Perform a Factory Default Reset on the AirPort Express as follows:
Power off the AirPort Express
Leave it off for 5 minutes or longer
Hold in the reset button first...and keep holding the reset button in for an additional 9-10 seconds while you simultaneously plug the Express back in to power
Release the reset button 9-10 seconds after you have plugged the power back in and allow a full minute for the Express to restart to a slow, blinking amber light
Temporarily, connect a spare Ethernet cable from your Mac to the Express
Open Macintosh HD > Applications > Utilities > AirPort Utility and reconfigure the Express to join the AirPort Extreme network using the new password that you established.
Once the Express has been reconfigured, you can disconnect the Ethernet cable.