For Professional subscription plans, account architects can update your company tax code via your Billing page. 

For Enterprise plans, you can update your company tax code (and any other invoice related information) by contacting our friendly accounts team.

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 change your tax code, in the "Billing details" box against "Tax code", click the "Update" link to update this information.