Log in to the calendar system, find the event on the "My Events" tab, and click on the event title. From this view, you can edit, delete, unpublish, copy, or cancel the event.
Delete vs. cancel: If your event has been cancelled, you have two options. Deleting the event removes it from the calendar entirely. Cancelling the event keeps the event on the calendar, but marks it clearly as cancelled. Choose the latter option if you want to communicate to audiences that the event has been cancelled.