If you need to update your billing email, you can do this directly from the Billing page by going to Settings --> Billing:
Once on the billing page, you will need to select "Edit Billing Information."
After you've selected the Edit Billing Information button, you'll be presented with a pop up where you'll be able to enter your new billing email. You will also need to re-enter your address and credit card information. You are asked to re-enter this information for security purposes and in the event you need to update your payment information.
To finalize the update, select the Update Payment button.