Perform a "factory defaults reset" according to the following:
- Disconnect the base station from its power source.
- Press and hold the reset button with a straightened paperclip, then plug the base station back in while continuing to hold the button for approximately 5 seconds until you see the three status lights glow.
- Release the button. The base station will factory default reset.
- Configure it using AirPort Utility.
Be advised that AirPort Extreme is very old, and will require an equally old version of AirPort Utility to configure it.
AirPort Utility for older AirPort base stations:
Leopard, Snow Leopard: AirPort Utility 5.5.3
Tiger: AirPort Utility 5.4.2
Panther: AirPort 4.2
Jaguar: AirPort 3.1.1
even older versions: http://support.apple.com/kb/HT1998