STEP 1) Go to the Calendar section
STEP 2) Click on the appointment you wish to delete (this will pop-open the "Appointment Details" window)
STEP 3) Click on the "Delete" button in the lower right-hand corner of the "Appointment Details" window
STEP 4) A warning prompt will appear that says "Are you sure you want to delete this appointment?" - click "Yes"
That's it! The appointment will be deleted.