Open the appointment from your bookings and use the cancel or reschedule control there. If the studio's policy allows the change at that point, it happens immediately and they are notified.
If you are inside the studio's notice window, the app will tell you and point you at the studio instead. That is deliberate: a late change is a conversation, and most professionals will work with you if you tell them what happened.
Cancelling late or not turning up may cost you the deposit or a cancellation fee, depending on that studio's policy. The policy is on the booking page and is set by them.
The single most useful thing you can do is cancel as early as you know. It is the difference between a slot somebody else can take and an hour nobody is paid for.