Open a web browser like internet explorer or Netscape. Enter the ip address of your router in the address bar of your browser. In the picture above the address bar has http://www.google.com in it. Just replace all of that with the ip address of your router. By default the ip address should be set to 192.168.1.254.
Click the Home Network button near the top of the page.
Click the Wireless Settings link near the top of the page.
When you're finished, click Save near the bottom of the screen to save your changes. And that's it! You're done!