The email that just went out asks, "Have you tried our new Markup & Material editor yet?" I can't seem to find such a thing. Nor a way to mark multiple items in order to edit their material/markup en masse. Where is this new editor to be? (Markup by material will be neat. It'll destroy my figure-out-markup Javascript, though.)
Go to your name at the top. Click on it. Go to number 3 (my shop) click on edit my shop. Go down to number 3 again and tada Product inventory management. Click that green button and you're there.