When I select send reminders for past due invoices and then click Send Reminder, can I edit the email before it is sent to customer?

Hello heidi14,  

If you are sending a reminder for one overdue invoice, you can edit the message in the pop-up which appears after selecting 'Send Reminder' on the drop-down against the invoice in the sales list. 

If you are batch-selecting multiple overdue invoices on the Sales tab to send, you will need to edit the reminder message before sending by going to the Cog Wheel > Account and settings > Sales > Reminders > Email message and edit from here.

Thanks :)