1. Search through here and do a Factory Default Reset.
2. Plug the Extreme into AC power and connect it to an Ethernet connector on the router. Wait until the indicator lamp is solid green.
3. Using Airport Utility app on a computer, an iPad or an iPod, locate the Extreme. Accept all default suggestions.
4. After you're up and running, you might wish to go back in to Airport Utility and set both a WiFi password and an administrative access password.