United Airlines on the App Store
https://apps.apple.com/us/app/united-airlines/id449945214
Easily manage your reservations on the app. - View and change your seat assignments, or enhance your trips by adding Economy Plus® seating, Premier Access®, United Club℠ passes, travel bundles and more. You can also change, cancel or rebook your flights right on the app, all through the “My trips” section. Continue using the app in the air.