How do i change an invoice from paid to unpaid?

I'd be happy to show you how to change a paid invoice to an unpaid invoice in QuickBooks Online, @tinkermaledublin. 

In order to have the invoice show as unpaid, the payment that was received needs to be deleted. Go as follows:

1. Go to Sales and select the Customers tab
2. Double-click the customer name and open the invoice. 
3. Click on the 1 payment hyperlink in the upper right-hand above the PAID payment status and hit the date.
4. Pick More button at the bottom and press Delete. 

To learn more about recording partial payment, applying credits and depositing funds in QuickBooks, please see these links below:

• Record invoice payments in QuickBooks Online
• Handle a customer credit or overpayment in QuickBooks Online
• Record and make bank deposits in QuickBooks Online

Let me know if you have a follow-up question about your invoice status or any other QuickBooks concerns. I'm always here to assist. Keep safe. 

You can delete it's previous payment, and then it will show as unpaid if paid, steps you can proceed with by opening an invoice and then click on below link on Paid invoice status and then delete previous payments.