Delete Stock Levels Before Stock Update Using Csv

I have a csv file from my supplier that i import to update stock levels. Sometimes product get discontinued therefore the quantity will not change in CS-cart. How can i bulk update the quantity in a certain category to ZERO before running the stock update? Thanks.

Products → Products → Gear icon → Global update

Thanks. I see that i have to manually add the products to the global update. I have over 6000. How can i select all in a category?

Note that our Categories Export/ Import module allows to export products from selected categories