Hello there, @claudeandcoy-gma.

I've got the steps that you need in changing the status of your paid invoice in QuickBooks Online.

You need to delete the payment that was associated with the invoice. This way, the invoice will be marked as unpaid, and you can apply the appropriate payment once received.

To do that:

1. Open your customers invoice and double click it.
2. On the Payment Status section, click the 1 payment link and tap on the Date hyperlink. 
3. Press on More.
4. Choose on Delete. 
5. Click Yes to confirm.

I'm also adding here some links that provides details about creating invoices and receiving its payments in QBO:

• Learn how to receive and record invoice payments in QuickBooks Online
• Learn how to create and send invoices in QuickBooks Online

Should you need additional assistance about invoices or with QuickBooks, please let me know. I'll be around for you. Take care.