Assuming you have the free native google shopping app, you go to Sales Channels>Google - then scroll down to the Product Feed section, to the right in the Product Status box, there should be a link on the right of that box, that says "Manage Availability". This should take you to a Google shopping bulk editor where you can change the product category and more in bulk fairly simply.
You can also do it via bulk import (export the products first to have the correct template set up, then save a separate copy and add in the google product details you want)
Extra tip: Use your arrow keys to switch between cells. AND/OR Hold shift to select multiple cells.