To cancel a flight with United Airlines, customers must contact Reservations at 1-800-UNITED-1 (1-800-864-8331). A service fee may apply depending on the type of ticket. Customers can also cancel a flight online. To do so, customers will be taken to the United Manage Reservations page, where they can enter their confirmation code, ticket number and last name. Then, select the ‘Cancel Flight’ option. All applicable refund amounts will be displayed to the customer.