How can I change a paid invoice to unpaid invoice

Hi there, nelblufood-gmail.

Thank you for visiting the QuickBooks Community. I'll be providing detailed steps to ensure that you can change a paid invoice to an unpaid one. Then, to appear the correct entry as closed/paid.

QuickBooks Online includes a Sales page that provides a quick overview of the status of sales transactions, such as open invoices and paid invoices. You'll need to find the entry from there and delete the payment associated with the paid invoice. This is to automatically change the status of the entry to open or unpaid.

Here's how:

1. In your QuickBooks Online (QBO) account, click Sales from the left menu and select the Invoices section.
2. Find the invoice paid mistakenly and click it to expand the page.
3. Click the Edit invoice button.
4. On the Invoice page, click the 1 payment made link under the Payment Status. Then, choose the date link.
5. In the Receive Payment screen, click More at the bottom.
6. Choose Delete, then click Yes to confirm.

Once done, the invoice will be marked as unpaid again. Then, you can now record the invoice payment into the correct entry so it will show as paid in the system. 

Lastly, you may refer to this article to see details on how to record a customer's refund based on various scenarios: Record a customer refund in QuickBooks Online.

Please know that you can always get back to this post if you have additional questions about this matter. I'll be sure to get back to you. Have a good day.