I wrongly applied payment to an invoice. How can I unapply a payment by check to an existing invoice?

It's good to see you here today, @thywillplumbing- . No worries, I'll help you in handling that transaction. 

To undo the received payment of an invoice, kindly follow these steps. 

1. In Sales, go to All Sales.
2. Select the payment you want to undo and click View/Edit.
3. On the lower right corner of the screen, click Clear Payment.
4. Hit Save and Close. 

If you want to convert an estimate into an invoice in the future, kindly refer to this link for further guidance: Convert an estimate into an invoice in QuickBooks Online. 

If you have any questions about invoices and other sales transactions, please let me know by adding a comment below. I'm more than happy to help.