1. Open the paid invoice.
  2. Select the Payments/Credits tab.
  3. Double-click on the payment record to open it.
  4. Click Void.
  5. Choose the correct Void date and Reversal post date.
  6. Click Void or Void and Purge*   
*If you are able to click "void" but are unable to click  "void and purge", you will need to post the reversal of the payment before you will be able to purge the payment. 

Note: If you mark the box Delete? the invoice will be deleted