For Professional plans, account architects can add/change billing contacts via your Billing page. 

For Enterprise plans, please contact our friendly accounts team to discuss updates to your billing contacts (and/or all other related invoicing/billing matters). 

Your billing page can be accessed by clicking on "Account settings" in the profile image pop-out menu, bottom left of the main navigation on the left hand side of the screen and then "Billing". 

To add/change billing contacts, in the "Billing details" box against "Billing contacts", click the "Update" link to update this information.
Please note: you can add up to five additional billing contacts. A billing contact will receive all billing notification emails (subscription renewal invoices, payment card expiry warnings etc).