Thanks for reaching out to us with such a great question, scnxxx!
To change your preferred email on your account, please visit my.xfinity.com and log in. From the upper menus, navigate to "Account" and then to "Settings". About half-way down you will see a section labeled "Contact Information" and you can use the "Edit" button to change your email.
Please give that a try and let us know if that helps!