As others have said, you can make and cancel reservations within the app as long as you more than 24-hours before the reservation time (e.g., if you have a reservation for 10:00 am on Tuesday, you have to cancel it before 10:00 am on Monday to use the app). I've been able to cancel a reservation online (via the website), within the 24-hour mark, but on the day before (so any time on Monday to cancel a Tuesday reservation). You can also call to cancel a reservation the day before.