If you can't join a class you can cancel it from your profile.
1. Go to "My classes" from the menu
2. Find the class you can't attend and click on "Cancel class"
3. Confirm your choice
As soon as the class is cancelled, it will be refunded in the "Remaining classes" under the section "Subscription" and can be rebooked at your convenience.
Please note: you can cancel a class until 1 minute before it begins. If you don't cancel the class and you don't attend it, it won't be refunded.