Hi,

Assuming you are already create the inventory adjustment before, you can do fast count or massive update by downloading the xls excel file and import it back.

First in your Inventory > Operations > Inventory Adjustment, select which product you may want to update, mark it, and export

Some people might missing one of the steps here. In order to be able to update the existing record, they also need to include the external ID into the export list this way

After that, edit your excel file. In this case, since you want to quickly put everything into zero you can assign zero to inventory_quantity cell. 

now, upload it back using import

Now you can see there's amount of difference. Try to put different number in your excel file, you will see it changes ;)

Kind Regards,

Altela (altelasoftware.com)