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

For all other paid 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 "Settings" in the navigation bar and then "Billing". 

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