Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Express icon, then click Edit
Click the Wireless tab at the top of the next window
Backspace/edit out the existing Wireless password and enter a new password
Do the same to Verify Password
Click Update to save the new settings and allow 40-45 seconds for the Express to restart
Tip....it is always a good idea to change the name of the wireless nework as well. If you do not do this, your computer stores the settings for the same wireless network with two separate passwords. As you might imagine, this can create confusion and conflicts.