Do you know what type of Airport it is?
Is it a small box, is it large and round?
Let us know
Depending on which version of the firmware you are using there are two ways to reset your router.
The first is to look on the bottom of the router and find a small button with triangular symbol above it, Press and hold the reset button until the light starts blinking rapidly then let go and the unit will reset after about 1-2minutes.
If your unit has an older firmware press and hold the reset button for a minimum 10 seconds and whenyou let go the Extreme will reset.
Once it has reset just open the Airport setup program from your Applications, Utilities and follow the steps to configure it.
Let us know if you have any issues.
For some reason I was thinking of the Airport Express, The extreme is beside the network connections.