With our 24-hour booking policy you can make changes to your reservation within 24-hours of buying your ticket without paying a fee. But there are a few rules: You must book your flight at least one week in advance. You must cancel or change your flight within 24 hours of the time you made your reservation. You can ask United Airlines for a refund to your original form of payment in case of an unplanned event due to which you can not fly within one year of the ticket purchase. Here are some of the unplanned events considered by United: