Select some item need to change the price -> click on Bulk actions -> select the necessary option -> enter the value -> click Confirm 
change item selling price: change the selling price of the selected items- > then enter the value >0 to increase the selling price or enter the value <0 to reduce the selling price
change item purchase price: change the purchase price of the selected items- > then enter the value >0 to increase the purchase price or enter the value <0 to reduce the purchase price
