Once you make a payment for a Bill, the status updates to Paid and creates a Payment Made. You have the option to remove the payment from that Bill.
To remove the payment, open the Bill and hover over the Payment.
When you do that, you should see an X next to the amount.
Click the X to remove the Payment. You will be asked to verify if you want to remove the Payment, click Ok.
On successfully removing the Payment, the Bill's status will update to Open or Overdue, depending on the date.
You will also see the available credits go up by the amount removed, you can use it for other Bills for this Vendor.
The Payment Made for this Bill will have $0 applied, you can delete it.