How do I mark a paid payment unpaid?

Click the refund button on the invoice or payment in order to change the status of the invoice from paid to overdue

To do this via invoice:

Go to the customer dashboard->invoice->click details on the invoice in question

Click Refund on the invoice

Enter the amount, select cash or check for refund method and click save

To do this via payments:

Go to the customer dashboard->Payments

Click details on the payment in question

Click Refund

Enter the amount, select cash or check for refund method and click save