Online flight changes are simple with United Airlines. This is how:
Open your United account and log in. Choose "My Trips." Next to the trip you want to modify, click "Manage." Next to the reservation you want to change, click "Edit." To Change your flight, click. From the drop-down choices on the right side of the screen, choose the new flight time and date, then click "Continue." After making your changes, click "Submit."
24 hours after making your request online, or 48 hours if you make it by phone or at an airport kiosk, your new itinerary will be accessible on your My Trips page.