On your Mac.......
Open Finder > Applications > Utilities > AirPort Utility
Click on the picture of the AirPort Extreme
Click Edit in the smaller window that appears
Click the Wireless tab at the top of the next window
Next to Wireless Network Name......Edit as needed as you wish
Next to Wireless Password.......Edit the password as you wish
Next to Verify Password.......Edit again to confirm the new password
Click Update at the lower right of the window and give the AirPort Extreme a full minute to restart
Once the AirPort Extreme has been updated, you will need to perform a Hard Reset on the AirPort Express by holding in the reset button for 7-8 seconds, releasing, and allowing a full minute for the AirPort Express to restart to a slow, blinking amber light status.
Then, the AirPort Express will need to be set up again to connect to your new network.