How do i reverse an invoice which was marked paid by mistake?

I'll help you change the status of the invoice, sandjolney.

Let's open the transaction and delete the payment. This will open it and make it unpaid again. Here's how: 

1. Open the invoice and click the 1 payment link in the upper-right hand corner of the invoice.
2. Click the date of the payment.
3. Uncheck the invoice to unlink the payment. You can also click Clear Payment to undo the transaction.
4. Click Save and close. 

I'll get right back in case you need additional help with your invoices. Thanks!