If an invoice has not been paid on time, you can easily create and send a payment reminder. This can be done in two ways. Both options are explained below.
Option A: Go to sales > payment reminders
Option B: You can create a payment reminder by from an existing invoice (unpaid). click the invoice for which you want to create a payment reminder. Click Action and then click Create payment reminder. Now you can easily create the reminder, make it final and send it.