In order to Batch Update Max Qty:
Navigate to the Inventory > Products Tab
Select the Category you would like to Batch Update from the Category Tree to the left
Select The Batch Update > Products Tab in the lower right corner
Check the top box next to the grayed out "Increment max_qty by"
Select the option you want, usually "Set max_qty to", and enter the amount you want it set to in the empty field to the right.
Select "Batch Update"