To cancel a flight on Frontier Airlines, go to www.flyfrontier.com and log in to your account. If you made your flight reservation with a ticket or confirmation number, go to "Manage Reservations" and input your information. Once you are on the Reservation Details page, find the "Cancel Flight" button. Follow the instructions from there to cancel your flight. Note that there might be cancellation fees due, depending on the fare rules associated with your ticket.