To accomplish this, please complete the following steps.

  1. Please set your grid to view the maximum number of items per page (1000) so that you don't have to repeat your actions too many times. 
  2. Then put a check mark next to the items you wish to change the quantity on. Or if it's all items on the page, you can use Multi Item Actions --> Select All on Current Page
  3. Then click Add Quantity (also under Multi Item Actions). 
  4. Select the new quantity that you desire per item. 
  5. Then click the Add Quantity Button
  6. Then click Force Reprice Selected (to force reprice the items that are already selected since you've now changed the default quantity setting for them and want to push the change through).