To remove a payment made against a bill, click on the 'Bills' tab and then the Options button on the right hand side of the bill you wish to remove the payment from.
From the drop down menu, choose 'View Bill Summary'.
In the payment details section at the bottom of the bill screen, click the Options button and choose 'Delete Payment'.
A slider screen will appear asking you to confirm that you wish to delete the bill. Click 'Delete'.
This will remove the payment and reclassify the bill as outstanding