From the United website
Request a change for Same-Day travel
You can confirm a seat on an alternate flight, to the same destination, on the same day of travel anywhere United flies within the United States and within four hours of the time of your request. If seats are available on an alternate flight, your change will be confirmed and you will be charged a $25.00 fee.
To request a Same-Day travel change, please visit an Easycheck-in® kiosk, or contact a Customer Service Representative at the airport check-in counter or call 1-800-United1 (1-800-864-8331).
Please see the following Same-Day travel change policy requirements:
Customers holding a discounted ticket in Economy class may confirm a seat on a flight for a $25.00 fee if available seats exist, instead of traveling standby.
Holders of First, Business, and full-fare Economy class tickets can make changes without paying the $25.00 fee as current policy allows.
Same-Day travel changes are available within the 50 United States.
Alternate flights must be scheduled for the same destination and to depart within four hours of your request. Flights outside of four hours do not qualify and the existing change fee policy applies.
Flight changes can be made at the airport with an Easycheck-in® kiosk, a United Customer Service Representative or with a Reservation Sales Representative by phone at 1-800-United1 (1-800-864-8331).
The $25.00 fee is nonrefundable and will be collected at the time the change request is made.
