The AirPort settings are saved on the AirPort Express. You change the settings with AirPort Admin Utility. I think you can "reset" the AirPort Express by uploading "Default Software" from the Admin tool, but I don't know if that will wipe all settings clean...
I just tried downloading the original software andd firmware to my Airport Express. Everything still seems to be the same. Setup Assistant will only recognize it if I choose 'change and existing . . . ' and if I try and update it from there, my wireless signal disappears (which is broadcast from a separate router??). I've about had it with the Airport Express thing. Is there a chance that it's just bad and nothing will fix it?
Don't use the AirPort Setup Assistant. Use the AirPort Admin Tool (should be in /Applications/Utilities along with the Setup Assistant). That gives you more options and bigger control and is just as easy as the wizard...
I've tried using both. I installed, or tried to install, the original software from within the Admin tool. Nothing I do is making the Airport Express work correctly. If at all possible, I'd like to wipe the Express completely clean as if new, because nothing is working properly and I'm at a loss of what else to do.
Important: This erases all settings and saved profiles. The settings erased include Access Control and RADIUS settings.
Follow these steps:
1. Unplug the AirPort Express from the power outlet.
2. Press and hold the reset button.
3. Plug AirPort Express back into an AC outlet (still holding the button).
4. Continue holding the button until you see the light flash green four times, which should happen after a few seconds.
5. Release the button.
As with a hard reset, the AirPort Express will not be visible in the AirPort menu item or AirPort Admin Utility for a short time. Remember that this whole process takes approximately 45 seconds to complete. You may then use the AirPort Express in its default state or reconfigure it using either the AirPort Express Assistant or AirPort Admin Utility. See the above hard reset section for information on the default settings.