Reset older airport express (802.11g)

You need to perform a Factory Default Reset to get the Express back to original settings, and then configure it again using AirPort Utility.

Pull the Express from power

Wait a few minutes

Hold in the reset button first, then continue to hold the reset button in for an additional 9-10 seconds while you simultaneously plug the Express back in to power

Release the reset button after the hold period and allow a full minute for the Express to restart to a slow, blinking amber light

The Express is now reset

The Express provides a default wireless signal with a wireless network name of Apple Network xxxxxx. Be sure to log on to this network first from your computer....no password is required....then open AirPort Utility to reconfigure the Express

This is not supported by Apple, but it does work for me when I try it.

It would be better to use another Mac running an earlier version of the operating system if you can to configure the Express. Otherwise, this is really your only option.