Apply a discount to multiple lines using the Apply Discount box that is enabled with the "Apply Multi Discount" setting in the User Preferences screen.


First, select the lines you want to apply the discount to. You can also use the checkbox in the header of the table to select/deselect all. 


Once selected, enter the the discount %, and click the "%" button. 



The discount will then be applied:



NOTE: if your line discount was selected to use a $ instead of a %, it will be reverted to the % selection when applying the discount.