You can cancel a booked class up to 1 hour before the class begins. Post the cancel window, you will not be able to cancel the class and we won't be able to any exception as well.
Tap on home tab
Scroll to the 'Upcoming' class, click on the session and choose 'cancel this session'
You can also cancel classes from your profile tab
Tap on profile tab
Select your Online GX Class Pack
Scroll down to future class, click on the options button to cancel a class
To reschedule that class simply book another available class after canceling the current class.