To delete an appointment click on the appointment from the calendar page.
On the calendar flyout that opens up, you'll see a little trash icon in the lower left. Click this to delete the session.
If there is an Invoice, Superbill, Psychotherapy Note, and/or Progress Note attached to that session, you will not be able to delete it until all of those items are deleted. You will get a pop-up window letting you know the reason(s) why you can't delete the session.
Click the links provided to open and delete those items, and then return to the calendar to delete the session.