In order to update the billing email address, you will first want to make sure that the email you want to change it to is verified on your account. You can do this by clicking the person icon and selecting My Settings. You will then see a link for Verify Address. On this page, you can enter in the new email that you would like to change to the billing contact. Once you enter the email, you should receive a verification email where you will need to click the button to verify. Once you have done that you can click on the person icon and go to My Account. From there you will be able to click Edit to edit the payment method, which includes the email address.