In accordance with united same day flight change policy, you may request a change beginning 24 hours prior to your scheduled departure. You may change to a flight that departs the day before or the day after your initial departure if the new one does so and it does so within 24 hours of the time your request is filed. If you checked luggage, you may still alter your flight the same day if you do it at least 60 minutes before the departure of your new flight.
Source Url:   

https://airlinespolicy.com/flight-change-policy/united-airlines-change-policy/