If the invoice is unpaid:
  1. In Records, Invoices, open the invoice
  2. Select the Payments/Credits tab
  3. Highlight the credit memo and click Unapply or Unapply All to unapply all credit memos associated with the invoice
  4. Save and close the invoice

If the invoice is paid (Unapply is grayed out on the Payments/Credits tab):
  1. In Records, Invoices, open the invoice
  2. Select the Payments/Credits tab
  3. Double-click on the check to open it
  4. Select Payment, Void from the menu bar
  5. Enter the appropriate information for the void and click Void or Void and Purge now
  6. Click the X to exit out of the check
  7. Highlight the credit memo and click Unapply or Unapply All to unapply all credit memos associated with the invoice
  8. Save and close the invoice