To edit an invoice that has been marked as paid:
1. Go to the customers profile and scroll to where you can see the Billing History.
2. Find the payment that has marked the invoice paid.
3. Click on the date to the left of the line item.
4. Enter a zero in the Amount box.
5. Click Save.
6. Go back to the customers Billing History.
7. Click on the date to the left of the invoice.
8. Change the amount in the Before Tax Amount box.
9. Click Save.
10. Go back to the customers billing history.
11. Enter the new amount in the Payment Amount box.
DO NOT MAKE ANOTHER PAYMENT.
12. Save the changes.