In page types, there is a page type named E-Commerce Transformations, which is where the order_content table and other items are stored. Click the transformations tab and you will see a list of them. Not sure exactly where you want to change this, so you may have to look around, but it should be in one of those. There is a tax section inside of the order_content transformation, which may be a good place to start. Also there are some ones dealing directly with taxes, which may also fix your situation.