For awhile I've been bedeviled by an older Airport Express that could not be accessed via password in order to update firmware. Again and again we tried to reset the password, without success. In searching through the Apple forums I found several people reporting this problem, with no definitive answers given. Most of the threads are too old for replies, so I decided to post here for the benefit of others in my situation.
Anyway, here's what I did: I finally tried the password "admin," and voila! Firmware promptly updated. At that point I was able to go in and change the password to one of my own. Anyone else with this problem might want to try the same thing. Good luck!