replacing my modem

Connect it to the coax line / outlet and power it up. Let it fully sync up with the system. Check that the front or the top panel indicator light(s) is / are steadily lit. 

Then connect a computer to it and open up a browser. You *may* see their *Walled Garden* modem self registration page. If so, follow the screen prompts. Have your mobile telephone number, your Xfinity username and password, or the account number and telephone number associated with your Comcast account handy.

If not, try calling 1-855-652-3446. That is their self-install device activation line.

If that doesn't work for you, you can call 1-800-comcast and tell a rep that you would like to have a new modem provisioned for service. Have the RF / CM / HFC MAC address and the serial number of it handy when you call.

There is also the option of using their Xfinity App for activating modems;

https://www.xfinity.com/support/articles/xfinity-app-activation?linkId=161715244