How do i change a paid invoice to unpaid?

Good day, murraypyke.

You'll need to unlink or delete the payment transaction from the invoice to change the status to Open or Unpaid.

Let me show you how:

1. Open the invoice and click on the payment made link under PAID.
2. To open the payment transactions, click on the Date link.
3. Remove the checkmark beside the invoice to unlink or click on More and choose Delete. 
4. Select Save and close.

I've attached some screenshots for you visual reference.

Just in case you need help with some various tasks in QuickBooks Online (QBO), you can also visit our Help Articles page through this link: Canada QuickBooks Community.

Keep me posted on how it goes by leaving a message below. I'm always here to help.