Changing and cancelling your flight is so easy your grandma can do it (but after reading this, you can be nice and offer to do it for her). Here are the detailed instructions.
Go to the American Airlines website and log in or click on the "Your Trips/Check-In" tab at the top of the destination page. You don't need to log into your account to see your next flight - you just need your name and check-in locator. If you log in, your next trip will automatically appear under the "Your Trips" tab.
Simply click on choose trip from the list of upcoming flights and select "View/Edit". You will then receive an overview of your route and other relevant details, and you will see two links at the top of the page:
Click on the appropriate link and American Flight Change will guide you to help you change or cancel your flight. You will receive an email describing your changes.
It's also worth noting that you might find that you cannot access your particular flight. This may be because you booked your flight with a partner airline or perhaps an online travel agency. Remember that you may need to contact the originating airline or agency to change your tickets.