Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Express icon and then click Edit
Click the Base Station tab at the top of the next window
Edit the name as you wish. Backspace out each password entry and enter new passwords
Click Update to save the settings
Note....this will change the device name and password of the Base Station, as you asked.
However.....if you really meant that you want to change the wireless network name, click the Wireless tab instead.