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"
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