Please see https://www.xfinity.com/support/articles/updating-your-mailing-address. You can't change this yourself, you need to have a Comcast rep make the change for you. If an Employee doesn't reply to your message here you'll have to contact a phone rep at the number on your bill or 1-800-Comcast, or a chat rep (see https://www.xfinity.com/support/contact-us). To visit a Service Center or Xfinity Store first check hours and locations at https://www.xfinity.com/support/service-center-locations.
The customer's inability to make this change/update to their account should be clearly indicated on the website. Being directed to links that don't do what they suggest (i.e. 'update your billing address') is annoying, time-consuming and aggravating.