Open Your appointments, choose the appointment, then Change or cancel. You can do this up to two hours before. After that, please call the practice on 0113 496 0000 so somebody can offer the slot to another person.
There is no penalty for cancelling, and it does not affect your record.
Cancelled slots go back into the pool immediately, so an earlier time may appear.
You need proxy access first. Set this up under Your details.
Nothing automatic. Somebody may call to check you are alright.
Hospital appointments are not managed here. Use the number on your letter.