The Edit overlay for V3 Installment Plans is very similar to the overlay you see when you Add an Installment Plan:
Once an Installment Plan has been created, you can change any field except the Account field. Click Show Advanced Options to reveal additional fields:
On any individual line item, any of the fields can be modified. Each line can be deleted by clicking the trash can icon on the right. You can also add a new line item by clicking Add line. You can add any number of line items for each month for more accurate revenue tracking. As line items are updated, the values at the top and bottom of the overlay will be updated to reflect the changes. When you are finished, click Save Installment Plan. If you do not need to make changes or to otherwise exit the overlay, click Cancel or the "X" in the upper-right corner of the overlay.