If the event/ride/tour allows cancellation, follow this procedure:
You will receive an email confirmation of the cancellation. If you do not, contact the organizer / tour leader to confirm.
Cancelling from the mobile app is even simpler. Just go to My Tickets, find your ride, then click Cancel.
Note: You cannot cancel yourself from a waitlist. If you are on a waitlist for a ride and need to cancel, send an email to the Tour Director, letting him know the event, and he will do it for you.