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. If you purchased a round-trip flight 1-888-829-1467 and need to change the first leg to a different day, your best bet will likely be to pay the change fee and book a new flight. If you only need to change the return portion of a round trip, however, it may be cheaper to book a new one-way flight than to pay the cancellation fee.