Welcome and thank you for taking the time to reach out to us here on the Community page, @stuartbhood.
You can delete the payment that was linked to an invoice to re-open the invoice again.
Here's how:
- Open the invoice that was marked as paid.
- Click on the payment link under the Payment Status section.
- Hit the date link to open the payment.
- Tap on the More button beside Print, then select Delete to remove the payment.
I've added these screenshots for your visual reference.
Let me know if there's anything you need assistance with. I'm always here to help. Have a good day!