There are three ways to upgrade a United trip after booking:
Sign in to your MileagePlus account, select your existing reservation and request the upgrade online.
Call United Reservations at (800) 864-8331.
Request the upgrade at the check-in counter or at the gate with a United representative, once you're within 24 hours of departure.
After making the request, you'll be placed on a waitlist that will be cleared as upgrades become available.