Inventory can be edited manually or through a Feed File.
To update inventory using the Manual tool, navigate to Products > View All Products:
Under "Inventory", adjust the inventory levels for the any products that are showing an incorrect inventory (for easy updating, you can use the blue arrows beneath "Inventory" to sort by lowest inventory, bringing these items to the top). Then, scroll all the way down to the bottom of the page and click Update.
To update inventory using a product Feed File, navigate to Products > Add/Edit Products with CSV. Here you can upload an inventory feed using a .csv file with the adjusted inventory levels in the “Quantity” column.
Create a simple .csv product feed file with two columns: "Unique Id/SKU" and "Quantity".
Once the feed is created and saved as a .csv, go to Products > Update Existing Inventory > Product CSV File:
Select your file and click Submit.