https://community.servicetitan.com/t5/General-Office/88-88-29-14-67-united-date-change-policy-within-24-hours-gt-gt/m-p/57119#M4128
It depends on the flight.
Most airlines charge a fee to change or cancel a flight in basic economy once you've passed the 24-hour window. The fee can vary widely by airline. However, other airlines have more flexible policies.
