6-2 from the main menu
Use this function to un-apply a payment from one or more invoices. Once complete, the payment will be un-applied and the once-paid invoices will no longer be paid.
When you do this, there is no need to re-export financial data, as no NEW money has changed hands. If you run an aged trial balance report for a prior date, the report should remain as it was when you first ran it.
To unapply a payment from one or more invoices, go to the Payment History tab of the customer record. Right click on the payment, then select "Unapply"