American Airlines Flight Change, at the airport (at the self-service kiosk or ticket counter) or call AA Reservations.
If you still want to try for a different flight, same-day standby lets you standby for an earlier American or American Eagle flight (with the same routine on your day of departure). Those traveling with American Airlines can change their AA flight reservations, but it incurs a service charge.
They can call American Airlines Reservations Number to change their ticket at any time of day at the following numbers:

English: 800-433-7300 or +1 833 584-08-69
Spanish: 800-633-3711
Hearing and speech impaired: 800-543-1586