Yes you can.
If you input data into your forecast that shows you collect £880 per month from your client (in this example, Paperchase), but for 2 months of the year, they pay you more or less (for example, additional income from a summer campaign), you can change the forecast to reflect this.
You need to go into your profit & loss forecast and click on the line item you wish to edit - in this case: sales - Paperchase.
Then you will land on this screen.
Click on 'Enter Values' below the client name.
Then simply type the new amount into the months where you need to change the data.
Now you can see the updated figures of increased sales for 2 months in summer: