I accidentally marked an invoice as paid, it has not been. How do I undo that.

Welcome to the Community forum, Michael.

I'm here to provide you with some information on how to update the status of an invoice in QuickBooks Self-Employed (QBSE).

Once an invoice is marked as paid, it cannot be reverted. You will need to delete the paid invoice and then recreate it. To achieve this, follow these steps:

1. Go to the left menu and select Invoices.
2. Find the relevant invoice that needs to be deleted.
3. In the Action column alongside the invoice transaction, click the dropdown icon, and then select Delete.
4. Confirm the deletion by clicking Delete again.

If you need to apply sales tax to the products you sell, you can refer to this article for detailed information on managing them:  Manually track sales tax in QuickBooks Self-Employed. 

If you have any more questions about managing your invoices, do not hesitate to get in touch by clicking the Reply button. We're here to assist you, Michael.