How to set a value multiple times within same update

Not sure I fully understand, but I suggest:

for (Revenue_Pipeline_Schedule__c s : scope) {

    Revenue_Pipeline__c p = pipelines.get(s.Revenue_Pipeline__c);

    // First clear all
    for (SObjectField f : RevenuePipelineScheduleUpdaterHelper.dateToField.values()) {
        p.put(f, 0);
    }

    // Then fill in the one
    SObjectField f = RevenuePipelineScheduleUpdaterHelper.dateToField.get(s);
    if (f != null) {
        p.put(f, s.Amount__c);
    }
}

Is there a reason you're using a batch here instead of a trigger? With a trigger, you could use the oldMap to reference what the record looked like before the edit. If you move this logic to a trigger, then each time a Pipeline Schedule is updated, you can "undo" the effect of its original amount/date on the Pipeline before adding the new amount/date.

Many of the other Answers seem to assume that there is a single Pipeline Schedule per Pipeline - but this will solve the problem without affecting the results of other Pipeline Schedules on the parent Pipeline.

Assume a parent with 3 children:

child1.Amount__c = 50;
child1.Date__c = 2017-01-01;
child2.Amount__c = 100;
child2.Date__c = 2017-01-01;
child3.Amount__c = 75;
child3.Date__c = 2017-02-01;

So,

Jan2017__c = 150;
Feb2017__c = 75;
Mar2017__c = 0;

If child2 becomes:

child2.Amount__c = 50;
child2.Date__c = 2017-02-01;

I assume the parent should become:

Jan2017__c = 50;
Feb2017__c = 125;
Mar2017__c = 0;

If called from a trigger on both insert and update, the following code should ensure this:

//Called from both insert and update, oldMap is null on insert.
//Assumes that Pipeline Schedules cannot be reparented
public void processRecords (Map<Id, Revenue_Pipeline_Schedule__c> oldMap, Map<Id, Revenue_Pipeline_Schedule__c> newMap)
    Set<Id> pipelineIds = new Set<Id>();
    Set<Id> pipelineSchedulesToProcess = new Set<Id>();
    for (Id key : newMap.keySet()){
        Revenue_Pipeline_Schedule__c newRecord = newMap.get(key);
        Revenue_Pipeline_Schedule__c oldRecord = oldMap == null ? new Revenue_Pipeline_Schedule__c(Amount__c = 0) : oldMap.get(key);
        if (newRecord.Amount__c != oldRecord.Amount__c || newRecord.Date__c != oldRecord.Date__c){
            pipelineSchedulesToProcess.add(newRecord.Id);
            pipelineIds.add(newRecord.Revenue_Pipeline__c);
        }
    }

    if (pipelineSchedulesToProcess.isEmpty()){ return; }

    Map<Id, Revenue_Pipeline__c> pipelines = new Map<Id, Revenue_Pipeline__c>([
        SELECT Oct2014__c, Nov2014__c, Dec2014__c, Jan2015__c
        FROM Revenue_Pipeline__c
        WHERE Id IN :pipelineIds
    ]);
    Map<Date, SObjectField> fieldMap = RevenuePipelineScheduleUpdaterHelper.dateToField;
    for (Id key : pipelineSchedulesToProcess){
        Revenue_Pipeline_Schedule__c newRecord = newMap.get(key);
        Revenue_Pipeline_Schedule__c oldRecord = oldMap == null ? new Revenue_Pipeline_Schedule__c(Amount__c = 0) : oldMap.get(key);
        Revenue_Pipeline__c pipeline = pipelines.get(newRecord.Revenue_Pipeline__c);

        //Subtract the amount that was added originally
        Decimal oldValue = pipeline.get(fieldMap.get(oldRecord.Date__c));
        Decimal valueToSubtract = oldRecord.Amount__c;
        pipeline.get(fieldMap.get(oldRecord.Date__c)).put(oldValue - valueToSubtract);
        //Add the new amount to the correct field
        Decimal newValue = pipeline.get(fieldMap.get(newRecord.Date__c));
        Decimal valueToAdd = newRecord.Amount__c;
        pipeline.get(fieldMap.get(newRecord.Date__c)).put(newValue - valueToAdd);
    }
    update pipelines.values();
}

I have tried to put simple logic to retrieve month value from the field.

Since every first day of the month, the process will be running, so from the schedule.Date__c get the current month.

Since, your field naming convention is Jan, Feb Mar,... etc, so create a map which holds the month integer and Name of the month.

Finally, compare the current month in the loop and update other month data with 0.

private static final Map<Integer, String> mapMonthValues = new Map<Integer,String>();
    static {
        mapMonthValues.put(1,'Jan');
        mapMonthValues.put(2, 'Feb');
        mapMonthValues.put(3,'Mar');
        mapMonthValues.put(4,'Apr');
        mapMonthValues.put(5,'May');
        mapMonthValues.put(6,'Jun');
        mapMonthValues.put(7,'Jul');
        mapMonthValues.put(8,'Aug');
        mapMonthValues.put(9,'Sep');
        mapMonthValues.put(10,'Oct');
        mapMonthValues.put(11,'Nov');
        mapMonthValues.put(12,'Dec');
    }

//Change for loop like this:
for (Revenue_Pipeline_Schedule__c schedule : scope)
    {
        Integer month = schedule.Date__c.month(); //retrieve the current month
        SObjectField field = RevenuePipelineScheduleUpdaterHelper.dateToField.get(schedule.Date__c);
        if (field != null)
        {
            if(fieldfield.getDescribe().getName().contains(mapMonthValues.get(month)))
            {
                pipelines.get(schedule.Revenue_Pipeline__c).put(field, schedule.Amount__c);
            }
            else
            {
                pipelines.get(schedule.Revenue_Pipeline__c).put(field, 0);
            }

        }
    }   
    update pipelines.values();

Try updating your second for loop in the execute(..) method to go through all the keys in your dateToField Map and set all of the fields to 0, apart from the one that needs to get updated. Like this:

for (Revenue_Pipeline_Schedule__c schedule : scope)
{
    for(Date dateKey : RevenuePipelineScheduleUpdaterHelper.dateToField.keySet())
    {
        SObjectField field = RevenuePipelineScheduleUpdaterHelper.dateToField.get(dateKey);
        pipelines.get(schedule.Revenue_Pipeline__c)
            .put(field, dateKey == schedule.Date__c ? schedule.Amount__c : 0);
    }
}  

However, I do have to say that your Parent object looks badly designed. Since you are clearing the previous values, why would there be a need to have a field for each month?

For this use case I would change the Parent object to have one currency field, and one date field to store the month of the currency. But I may not be seeing the bigger picture.

UPDATE

OK, so I think I understand your requirement now.

If it is possible I would add two new fields to your child object:

• Old_Amount__c
• Old_Date__c

Now, update your batch

global void execute(Database.BatchableContext BC, List<Revenue_Pipeline_Schedule__c> scope) {

    Set<Id> pipelineIds = new Set<Id>();
    List<String> fieldNames = new List<String>();

    for (Revenue_Pipeline_Schedule__c schedule : scope)
    {
        pipelineIds.add(schedule.Revenue_Pipeline__c);

        SObjectField oldField = RevenuePipelineScheduleUpdaterHelper.dateToField.get(schedule.Old_Date__c); 
        SObjectField newField = RevenuePipelineScheduleUpdaterHelper.dateToField.get(schedule.Date__c);

        if(oldField != null){
            fieldNames.add(oldField+'');
        }       
        if(newField != null){
            fieldNames.add(newField+'');
        }
    }

    //This will retrieve all required fields
    String query = 'SELECT ' + String.join(fieldNames, ',') +
        ' FROM Revenue_Pipeline__c WHERE Id IN :pipelineIds';

    Map<Id, Revenue_Pipeline__c> pipelines = 
        Map<Id, Revenue_Pipeline__c>((List<Revenue_Pipeline__c>)Database.query(query));

    //now that you have all the fields for pipelines, 
    //go through the updated children (scope) and 
    //subtract the old value from parents, and 
    //add the new one   

    for (Revenue_Pipeline_Schedule__c schedule : scope)
    {
        SObjectField oldField = RevenuePipelineScheduleUpdaterHelper.dateToField.get(schedule.Old_Date__c); 
        SObjectField newField = RevenuePipelineScheduleUpdaterHelper.dateToField.get(schedule.Date__c);

        Revenue_Pipeline__c p = pipelines.get(schedule.Revenue_Pipeline__c);      

        if(oldField != null){
            //subtract the old amount from the old month field
            p.put(oldField, p.get(oldField) - schedule.Old_Amount__c);
        }

        if (newField!= null)
        {    
            //add the new amount to the new month field   
            p.put(newField, p.get(newField) + schedule.Amount__c);
        }

        //update the old field values on the child
        schedule.Old_Date__c = schedule.Date__c;
        schedule.Old_Amount__c = schedule.Amount__c;
    }   

    update pipelines.values();
    update scope;
}