Here's what to do in case you'd like to change an invoice that you already marked as paid, but later found out that the customer's check bounced for example.
Delete Payments For the Invoice
Before you can edit a paid invoice you need to remove the payments from your invoice.
IMPORTANT: If you remove a payment that was actually paid by your customer please make sure to add the removed payment amount as credit to the customer's credit balance.
To remove payments first click 'Payments' on the invoice detail page:
And then remove the payments belonging to the invoice:
Once you removed all payments from an invoice you will be able to edit it again and re-send it to your customer.