Make a call to American airlines at 800-433-730 to change your reservation. The time of the departure of your new flight reserved must be within 12 hours of your call. A reservation change fee will be applied.
There will be a self-service kiosk in the airport which you can make use of, if you are already in the airport. You can make the change in the reservation by yourself and print the new boarding pass from the machine. Reservation change costs are applied and can be paid via credit card.
If you are already in the airport you can move to the American airline counter to ask for help. They will provide the details of flights to your destination in the next 12 hours. You can choose the comfortable one and pay the reservation changes fee.