Hi, alanahwray
Changing the invoice from paid to unpaid is currently unavailable in QuickBooks Self-Employed. As a workaround, you can duplicate the invoice and delete the paid one.
Here's how:
- From the Invoices screen, locate the paid invoice and click the drop-down arrow under the Action column.
- Choose Duplicate.

- Review the Invoice details and tap Save.

- After saving the duplicate invoice, delete the paid one.

- Hit Yes to confirm.

To know more about how invoicing works in QuickBooks Self-Employed, check out this write-up: Create invoices in QuickBooks Self-Employed.
Feel free to drop me a reply below if you have any other invoice or QuickBooks concerns. I'll zip right back to help you out.