Q129: How can I modify or cancel existing reservations?

You can do this on the reservation details page. You can open it by clicking on the desired reservation ID (in reservations, schedule or search tabs of the admin panel).

To modify the status of the reservation (e.g. confirm it) or cancel it, use one of the buttons below the reservation details. If you want to change the duration or start time, click 'modifications'. Here you'll be able to reschedule the reservation. There's also a quick way to move a reservation to a different time or resource explained here.