When you mark an invoice as Paid, QuickBooks also creates a payment associating it to the affected invoice. We can delete the payment and the invoice will go back as Open. Here's how to delete the invoice payment.
Open the affected invoice.
In the Invoice window, click the 1 payment made hyperlink below the PAID payment status.
Click the payment date and it will route you to the Receive Payment window.
Click More at the bottom menu and select Delete.
Select Yes to confirm the action.
After that, you can already send reminders to your customers.
Here are some additional links related to reminders that you can use in the future: