You have up to 4 hours before the start of your session to cancel a reservation.
Alternatively, you have one hour to cancel a reservation from the time you make it (provided that this hour has not yet begun).
In both cases, all your credits will be returned to you.
Modifying a reservation is not possible: you will have to cancel your reservation and make another one.
To cancel a booking, go to 'My bookings'. Find the booking in question, press +. Scroll down to 'cancel my booking'. Press + and then cancel my booking. You will receive your return credits immediately.