Two things that would want to do
1) Perform a Hard Reset or Factory Default Reset to erase the current network settings that might be stored on the AirPort Extreme. The Apple Support document inked below will provide all the details on this.
How to reset your AirPort base station - Apple Supporthttps://support.apple.com › en-us
2) Update the firmware if it is not already updated. The most current firmware update is different depending on what model of the AirPort Extreme that you have.
If the AirPort Extreme is the most recent "tall" or "tower" shaped version, the current firmware is version 7.9.1. If the AirPort Extreme is "flat" or "square" and shaped like a small white pizza box, then the most up to date firmware for that version is 7.8.1.
That Apple Support document linked below will provide details on how to do this.
https://support.apple.com/en-us/HT201519
If the AirPort does not come with a printed Setup Guide, you can find the manual for the AirPort here:
https://support.apple.com/airport