If you want to change time of your appointment, click it and in the bottom left corner pick Move button. After confirmation you will see "Appointment in clipboard" notification in the left calendar legend. There is a Clear option if you changed your mind at this point and would rather keep existing appointment and proceed with booking another one. Finally you select new appointment time and movement is done.
You can move your appointment within 7 days.
You can not move an appointment if there is less than 24 hours until its start.