Hi Peter,
Thank you for your inquire.

How do customers change their own appointments? 

At the moment they can drag and drop their own appointment in the Calendar page, so they will need to have WP credentials to login to WP.

Or is there a page that I can see that is a front-end page where customers can change appointments?

For our next major update we do have in plan to implement Front end booking cabinet for customers, where this will be available.

Best regards.