Newer editions of Cisco routers have a Quick Setup menu. Under this screen, you can make changes to your Wireless network.
If you don't have Quick Setup, look for Wireless or Wireless Security.
Now it's time to create your new WiFi password. Look for a WiFi password field. It may be referred to as a Network Key. Enter the new password and save the changes.
When the information is saved, disconnect from the router and test the new password.