You are able to see all upcoming registrations from the dashboard tab on your account.
Besides each class you have a leave option. If you click this you can unregister from the class.
Note. You can leave a class at any point before the class starts. If you are unregistering after the cancellation cutoff, you will be able to do so, but you will not receive a refund/your membership usage back.
You can also leave a class direct from the schedule tab on your account.
Just head to the day you are registered, and click leave.