What is the best way to reverse an invoice that has been marked as paid by mistake?

Hello Anth131078, 

Welcome to the Community page, 

What you need to do is just go in to the payment and then at the bottom of the page you will see a button that says more click on that and then select the option that says delete, dont use the void it needs to be the delete one if it was done my mistake. This will then make the invoice open again so when it does actually get paid you can then mark it as paid.