Assuming that you are using the Lion or Mountain Lion operating system on a Mac......
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Extreme icon, then click Edit in the small window that appears
Click the Wireless tab at the top of the next window
Edit the Wireless Network Name as you wish
Click Update at the lower right to save the changes
Wait a full minute for the AirPort Extreme to restart