You can either :-
[1] Go to the home page, click Finances | Payments and double click on a payment
[2] Go to the Patient Editor, Account Balance screen and double click on a payment
[3] Click the ‘payment status’ label on the invoice editor (which says “This invoice has been paid in full”) and then click the payment you wish to amend
Any of the above will bring the up the payment editor, from where you can delete or edit a payment.