https://support.hammerhead.io/hc/en-us/community/posts/29744256068763-888-829-14-67-How-long-do-I-have-to-change-my-flight-on-United

Well, United Airlines allows passengers to change flights up to 24 hours before their scheduled departure time, for free and with no penalties. After the 24-hour period, United Airlines Change Fees will be $200 to change the date and time of their flights.