It's easy to adjust the existing "Stock" level for a specific SKU.
Click on the Products tab.
Next, select Inventory from the sidebar on the left.
On this screen, you'll see all the existing stock levels for your account. (If you haven't set up your initial stock levels yet, check out this article.)
You can modify the values in the "Stock" column by adding units, removing units, or setting the stock count to a certain amount. Click on the appropriate symbol in the "Actions" column to get started:
When you add, remove, or set stock, you can add a note to help you remember why you made the adjustment. Note that when you add stock, you can also specify your cost for those additional units.
These functions, including a history of changes, are also available via the Inventory sub-tab on the Product Detail window.
That's it! Making an adjustment to a single stock level is really simple, but it would be a lot of clicks to update multiple SKUs in this way. To update stock levels for multiple SKUs in a bulk process, please refer to this article.