If you click the appointment you want to delete, a window will open on the right hand side, click on the "... " in the top right hand corner of this window and then click "Cancel booking":
Once you click cancel you will see the below pop-up where you need to click "Confirm":
This appointment has now been cancelled and removed from your calendar.
If you would like this to send a confirmation of this appointment cancellation to the patient, tick the box "Email patient confirmation?"