Hi there,
You've come to the right place for help. Don't worry, that's an easy fix. I'll be happy to assist with marking the invoice as unpaid.
To do this, click Sales > Invoices > locate the invoice in question > click on where it shows "Payment made on **/**/2019" > once you're on the payment screen click More > Delete. After doing this, your invoice should show as unpaid.
Give this a try and let me know how you make out. In the meantime, check out these helpful resources:
How to void or delete an invoice or other transactions
How to void or refund a credit card payment
Feel free to ask questions. :)
