This process will only work for bills which have been created via the Purchases menu (by default the prefix will be PUR). This will not work for PAY prefixed bills, these bills will need to be Voided completely and re-explained.
Navigate to the Purchases > Bills > All menu.
Click on the appropriate bill ID.
Scroll down to the Payment history table and hit the Delete link next to the appropriate payment ID.
Please note: Deleting a payment will remove it from any other bills to which the payment is allocated.
Use the Delete payment button to delete just the payment or use the Delete all button to delete the payment and any bills to which the payment is allocated.
Please note: If any of the bills have other payments allocated to them, it will not be possible to use the Delete all button.