You must first start in List View (use the View toggler in the upper right hand corner)
1) Check the boxes of the products you'd like to edit together
2) In the dropdown menu select Mass Edit
3) The Mass Edit panel allows you to Apply, Replace, or Clear any of the attributes to all of the selected products at once.
Note: Type and Category values can also be deleted permanently from this panel by hovering on the value and clicking the X which appears to the right of the value.