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. If you have a Basic Economy ticket or booked your flight using Money + Miles, you can't make changes but you can cancel for a full refund. Do I get credit from United if I cancel my flight? If you've had to cancel or change a United... https://community.tableau.com/s/idea/0878b000000ExfZAAS/detail