All the Buyers related information including the Tax details is stored in Attendee/Buyer List. Click the manage Icon for the Buyer whose tax details you want to update. On click of Manage Icon, you will see a dropdown, select "Edit Tax Details" options and you will see the information provided by the Buyer. Make the necessary changes and save it. Post this, ask the Buyer to click the "Download Invoice" button from the confirmation mail and the buyer will get the updated invoice.