You might also reboot your Mac and run Air Port Utility with no other applications running.
There should not be any interference from other applications, but the possibility is always there.
If for some reason the firmware update has been corrupted when downloading or is inaccessible to the utility, then you may get an error.
Go to the /username/Library/Application Support/Apple/Air Port/Firmware/ folder and remove any files within this folder.
In the months that followed, Apple updated the Air Port Express again, changing the form factor (it looks like a little white Apple TV now), adding simultaneous dual-band support, and giving it model number A1392.