Hi there, @julie-trumbullca. 

Welcome to the Community. We can delete the payment linked to the paid invoice to mark it as unpaid. Let me show you how to do it: 

1. Go to the Sales menu, then select click the Invoices tab. 
2. Locate and click to open the invoice.
3. In the Invoice window, click on 1 payment link at the upper right hand below the PAID payment status, then click on the link for the date.
4. On the Receive Payment window, click More at the bottom.
5. Click Delete, then select Yes to confirm. 

You have just marked the invoice back to unpaid status. 

For future reference, read through this article: Void or delete transactions in QuickBooks Online. It'll help you learn to decide when to delete or void transactions in QuickBooks. 

Feel free to visit again if you have other questions. I'm always around to help.