It depends on the equipment, and being a Plus member does not change the rules. They are the same for everyone.
On free Multisport equipment, a booking cannot be cancelled at all, not even before you use it. It has to be ended at the station, with the return photo, even if you never took the equipment out.
On paid equipment, cancel it in the app up to 24 hours before it starts.
A session that has already started is never cancelled. Go to the station and end it by returning the equipment in the app.
Full details, including what to do less than 24 hours before a paid booking: How to cancel your reservation.
Once that is done, you can book a new session right away.
Have a good session.