I took mine to my nearest Xfinity Store. If that is not an option, you might start with chat - I got mine that way.
Bottom of the page where it says Ask Xfinity:
adding... do you have a recent model of Comcast gateway/modem with wifi built-in?
Hello chrisjohns03, thank you for reaching out to us here with this concern. I'd be happy to get a new Xi6 sent out to you. To get started could you please send me a private message with your full name by clicking on my name (ComcastAmir) and then click "Send a message"?