There are 2 ways to cancel your bookings.
First, is to contact your gym administrator or any staff and cancel your booking and removed you from session from the app or the admin portal.
2nd is for you to log in to your account and "Unbook". You may do it by tapping "Gym schedule" from the home screen of the app. You will then see your gym gym calendar, tap on the date/session that you booked then Unbook.