From settings we can do:
Thanks for writing us in.
Apologies for the confusion.
The billing contact is the "additional contact" of the customer for which the “billing email” checkbox has been enabled as in the screenshot below.
The additional contact billing email is different from the Customer’s billing email and the Subscription shipping email. You can add the additional contacts using “add contact” option on the customers detail page.
Thanks for your reply.
I was asking if there's a way to update this email for a customer/subscription from the API. Is it possible?
Thanks for you email!
Yes it is possible.Chargebee allows you to add your contacts via an API. The additional contact of the customer can be added using the API shown here add a contact.
Hope this works for you.