The first thing you are going to need to do is locate a Mac running an older operating system like Leopard, or Snow Leopard, or a PC.......since Apple has shut off support of the older AirPorts in Mavericks OS X (10.9.x).
For the above Macs, download and install AirPort Utility 5.6.1 for Leopard/Snow Leopard - Support - Apple
For a PC running XP or Vista download and install AirPort Utility 5.5.3 for Windows - Support - Apple
For a PC running Windows 7, download and install AirPort Utility 5.6.1 for Windows - Support - Apple
Then you will need to perform a Factory Default Reset on the AirPort Exteme. Use this Apple support document for reference:
Resetting an AirPort base station FAQ - Support - Apple
When you have the software installed on a computer, and you have completed the factory default reset, post back for more instructions.