Hi there, dpatten.

Correcting the paid Invoices as unpaid is easy. I'll show you how:

1. Go to Sales and choose Invoices.
2. Select the Invoice transaction you want to mark as unpaid.
3. Click the Payment made hyperlink in the upper right corner.
4. Click for the date you marked it as paid.
5. Click Clear Payment.
6. Select Save and close.
7. A prompt message will appear, click Yes.

Please see the screenshots below: 

Please let me know if you have additional questions. I'd be glad to answer anytime.