If you have indeed already changed it you will need to set the device back to factory defaults using the recessed reset button on the rear before the above advice (default username and password) will work.
If you are using one of the Xfinity Wireless Gateways, then look at the sticker on the device. If it's been changed then log into the gateway and find it there. If you can't login, then call the gateway support folks as has been mentioned multiple times in this thread.
If you don't have one of these gateways, then why are you on this thread? Start your own thread and please, give complete details, exact router, exact config, OS on the computer, "ipconfig /all" output on the computer, etc.
Log in..people. Its there.. u can reset your device..then login..with default...u can get network key from there... you cant just come in a thread and say...I NEED HELP WITH MY PASSWORD, and supply no details....