How do I reverse a payment received to the wrong customer invoice and make that invoice open again in Quickbooks Online?

Thank you for posting here in QuickBooks Community, @Cindi_I. I'll provide information on how to correct the payment that was applied to the wrong customer.

Since the payment was not applied to the correct invoice, let’s unlink or delete the payment transaction for the invoice to be tagged as Open. Then, create again a receive payment and link to the correct invoice.

Here's how:

1. Go to the Sales menu.
2. Select the All Sales section. 
3. Locate and click the payment transaction.
4. You'll be routed in the Received Payment page.
5. Tap More at the bottom and choose Delete.
6. You'll be prompted with Are you sure you want to delete this? Hit Yes.

See this article for other ways of correcting payments: How to fix a payment applied to the wrong customer invoice.

Once done, you can now create a payment and apply it to the correct invoice. Please check this article to see the steps: Record invoice payments in QuickBooks Online.

That will automatically change the status of the invoice to Open (or unpaid). Don't hesitate to add a comment if you have follow up questions. Have a lovely rest of the day!