You can send an invoice reminder emails several ways:
From the Invoice page
Click on the Invoice Menu button and select "Send Reminder"
From the Invoices List page
You can filter by all Open and Overdue invoices on the Invoices List page and fire off your reminder emails by clicking the down angle button under the Actions column.
From a Production's Invoice List page
Similarly, you can navigate to the Invoices tab of any Production, filter by Open and Overdue invoices, and fire off your reminder emails by clicking the down angle button under the Actions column.
You can enable/disable invoice reminders for all new invoices in your Invoice Settings.
You can enable/disable reminders for a specific invoice on that invoice by clicking the Reminders Edit button.
Which reveals that invoice's reminder settings: