Can I change the status of a selling invoice from PAID to UNPAID?

delete the payment, and the deposit first if the money was deposited

Yes, you can, @cristina-galeott.

In addition to Rustler shared, here's how to delete an invoice payment in your QuickBooks Online (QBO):

1. Go to the Sales menu.
2. Select the Invoices tab.
3. Locate the invoice you want to change status and select View/Edit from the drop-down menu under the ACTIONS column. 
4. Click the 1 payment made link and click the date link to open the payment. 
5. From the Receive Payment page, unmark the invoice to unlink the payment. (You also have the option to either delete the payment or apply it to another invoice).
6. Click Save and close. 
7. And click Save as a credit to save the payment as a credit.

I also recommend getting in touch with your accountant for guidance in deleting the payment. That way, we ensure the accuracy of your books after deleting it.

You can also check these great articles for reference in case you need to record a credit card payment and delayed credit: 

• Record credit card payments
• Enter and apply a credit memo or delayed credit

Don't hesitate to leave a comment below if you have other concerns or questions. The Community is always here to back you.