Hello,
All you need to do is Select ¨Payments of Accounts Payable¨ from the ¨View¨ menu, then click on the payment you would like to reverse or change to highlight it.
From here, it depends on what you wish to do, you can either delete a payment by clicking on the delete button on the toolbar and then click ¨Delete¨again to confirm or you can click on Edit and change the amount of the payment.
I hope this helps!