When creating a new product (from the Product View tab under Inventory),  you can toggle the “Taxable” switch to either be taxable (blue) or non-taxable (grey).

You can also edit an existing product by clicking on the Edit icon (box with pencil) under Product View in the Inventory section.

NOTE:   You can only edit products that are in your master channel (usually your accounting system).  Products from connected sales channels are not editable.  You will need to edit these products directly in your sales platform software.