I incorrectly marked an invoice as paid when it hasn't been. How do I change this?

Hello Adam130, Welcome to the Community. We can certainly guide you with how to remove a payment from an invoice.

Let me share the steps on you how can do this in the online version within the invoice. Here's how:

1. Click the 1 payment made link in the upper-right hand corner of the affected invoice. This is just below the PAID status.
2. Click the date of the payment.
3. Once the payment is opened, click More at the bottom (on the black banner) and select Delete.
4. Click Yes.

Please take note that deleting a transaction in QuickBooks Online cannot be undone. 

Any questions let us know, we'd be happy to help further.