Cancel your class reservation and get your credit back.
Open the Schedule and find the class you booked, or go to "My Bookings" to see all upcoming bookings. My Bookings also lists your appointments and the events you booked, marked "Appointment" or "Event" — tap one to open it (appointments are changed or cancelled on My Appointments, events on the event's page).
Click on the booked class, then click "Cancel Booking".
Your credit is refunded automatically. If someone was on the waitlist, they'll be moved into your spot.