You're not able to edit the amount of the payment, so you would have to delete it and re-enter it. You can however modify the description (just click and type).
Advanced figures aren't really used in QuickFile (i.e. there's currently no forecasting functionality), so really you only need to enter data as the invoices arrive. I wouldn't recommend entering estimated figures as they would affect your P&L report, and VAT (if you're registered).
Hope that helps!