In order to either reinstall or downgrade the AirPort's firmware you need to be able to access it. Typically during a firmware update failure a "factory default" reset should get you back to accessing the base station. Note you may have to perform this type of reset several times to get it to take.
If, after attempting multiple resets, the AirPort does not restart it may be "bricked." The only option then would be to contact Apple directly for what your options are.