On the Debts screen you will find on the ledger, to the right side of the screen, a small link icon shown next to the mortgage. Click this link and you will see a mortgage payment expense is linked to the debt. You may click this link to go to the payment expense or go directly to the Expenses > Basics screen and locate it.
This linked payment expense is created by the software automatically whenever a debt is entered into a plan. You enter the basic terms of the debt (the required payments) on the Debts screen and can then edit the linked debt payment expense, on the Expenses > Basics screen, to schedule overpayments or to make future adjustments in payment amounts.
To show over payments on any debt, simply edit the amount of the payment expense on the Expenses > Basics or you might use that screen's Step Up / Step Down facility to schedule a future increase (or decrease) in payments, if needed, using events.