On the Xfinity homepage hover your mouse pointer over the location it's showing now. You will see a small window and next to the location is a small pencil icon. Click on that, and on the landing page at the top is a field in which you can type the zip code of the location you want. Put it in and hit enter.
How to keep my selection of where I want my weather information to be permanent on my start up page was not given
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I have tried this several times but it still reverts back to (in my case) Los Angeles. I live in the SF bay area.
Do you have cookies enabled for Xfinity?