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:

1. Head to the Invoices menu, then locate the paid invoice.
2. Click the Action dropdown, then hit Duplicate.
3. Ensure that the invoice date, due date, and number are correct.
4. 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.