Made error and marked an invoice paid. How do I mark unpaid?

Thanks for dropping by the Community space, @swatm-gainusa-co. 

Editing your invoice status is a breeze, and I'm here to help you. Let’s go to the Invoice page to change it.

Here’s how:

1. In your QBSE company, tap the Invoices menu on the left panel.
2. From the list, choose the entry you’re working on.
3. Then, click the View drop-down under the Action column to select Mark as unpaid.
   

For future reference, this article provides an overview of how to customize your invoice and steps to activate the payments option: Create invoices in QuickBooks Self-Employed. 

Additionally, I've included an article that'll help you track how much sales tax you've collected throughout the year: Manually Track Sales Tax in QuickBooks Self-Employed.

Please let me know how it goes for you. I'm always around to help whenever you needed me. Have a good day!