Hi there, Gary. Marking an invoice as paid by accident can happen. Let's resolve this together.
In QuickBooks Self-Employed, once you mark an invoice as paid, reverting it to unpaid isn't possible.
In your case, we can create a duplicate of the paid invoice by following these steps:
- Head to the Invoices menu, then locate the paid invoice.
- Click the Action dropdown, then hit Duplicate.
- Ensure that the invoice date, due date, and number are correct.
- Once done, save it.

After completing these steps, you will find an open invoice in the list. Please delete the transaction that was accidentally marked as paid.
I'm adding a sample screenshot for visual reference.

This thread will stay open for any additional questions. We're always here to assist you.