How do I change an invoice that has been marked as paid but has not been paid

You can unlink the payment of the invoice, danielbrash79.

If the customer has available credits and you turned on the default setting to automatically apply credits, an open invoice for that customer will be marked paid. To mark it as open, you can follow these steps to unlink the payment. 

1. Click the Sales menu and select Customers. 
2. Look and click the customer's name.
3. Locate and select the paid invoice. 
4. At the upper-right of the Invoice page, click the 1 payment made link.
5. Click the date to be routed to the Receive Payment page.
6. At the bottom menu, click More and select Delete.
7. Confirm the action by clicking Yes.

You can check the invoice status in your customer's transactions list. Further, if you want to disable the auto-apply credit option, let me show you how: 

1. Click the Gear icon and click Account and settings.
2. From the Advanced menu, click the Automation section.
3. Toggle to turn off the Automatically apply credits.
4. Click Save and then Done.

To help you manage customer payments, you can check out these articles: 

• Record invoice payments in QuickBooks Online
• Handle a customer credit or overpayment in QuickBooks Online

Just keep me posted if there's anything else that you need help with. Take care!