Before you change your user ID, try marking messages as Spam if they get to your Inbox folder. It helps to train the filters. But if you really want to change your email address, see here-----------------
Yes, you can change the user ID-----that portion of the email address to the left of the @ symbol. On the Xfinity home page, click on "My Account" at the top. Sign in again. On the top of the landing page, click on "Users". The account name should now show. To the right, click on "Edit", and follow the prompts.
Caution-----once you change the user ID, you CANNOT get the old one back. So make sure the new one is really what you want.
Taking Latoque's warning one step further, it sometimes makes more sense to create a new user (you can have up to 7 email addresses on your account) instead of changing. Realize that if you change your address ANY email sent to the old email address will bounce, so you risk not getting any legitimate mail you are looking for. If you create a new user you can take the time to transition your addresses without fear of missing important emails, or someone else taking your old address after a time and getting emails for you.
Just my two cents