The latest info on United Airlines mobile apps
There is an iOS app from United Airlines which will work on your iPhone, iPod, or iPad.
For United Airlines MileagePlus members, you'll be able to manage your account from the app, though using their website will no doubt provide more functionality
United Airlines has an Android app that works with most Android tablets, phones, and some Android Wear devices. Perhaps standard practice these days, the Android app offers the same functionality as the iOS app from United Airlines.
Also take a look at United Airlines's website by clicking here — you should be able to do what you need there.