If you have iOS though, you can cancel or reschedule using Apple's Apple Store.app. It's not obvious how ...
- Tap Stores
- Find store where you have reservation.
- Click Store then Genius Bar. From here you can create a new appointment, or cancel or reschedule an existing appointment.
I don't think there's an Apple Store.app equivalent for the desktop, so, like iMessenger, this is strictly an iOS service.