Thanks for reaching out to the Community with your question. I appologize you are unable to edit the billing email address associated with your account. If the email you want to update to is not showing as an option, I suggest to verify the email in your account settings first. Verifying this new email address will allow it to show in the drop-down of options in your email settings when doing any updates. Doing so will then allow you to choose this new email as your billing email address.
If you are still unable to update your billing, please email us at social_support(at)constantcontact(dot)com with a reference to this post and your username so we can troubleshoot further with you. You can recieve help updating your billing information from a memeber of our Billing Support Team at the hours and numbers listed here.