Hello,
Press and hold the reset button for about 5 seconds, until you see the status light on the base station flashes amber rapidly. Then release the button. Wait about a minute for the base station to finish restarting. Open AirPort Utility, which is in the Utilities folder of your Applications folder.
Thanks.
For More go to:-
Apple AirPort Setup
Reset AirPort Extreme
Apple AirPort Extreme Setup