In the Additional Tools menu in the Builder, there is a section called Taxable Labor Options. This tool provides a quick, easy way to apply or remove tax on all items in a project.
First, make sure that you are in the Builder section of your project. Then, click on Taxable Labor Options.
In the overlay that appears, you are given the option to either select or de-select all items to be taxed. Once you are finished, click Update and your preference will be applied to all of the items in your project.