Once you’re logged in, open the Product you wish to edit.
By default, the option to Manage Stock isn’t turned on. If you go to a product and it isn’t turned on yet, check the Manage Stock box.
After you’ve checked Manage Stock, a field will appear for you to enter the Stock Quantity.
Also be sure to select that it is In Stock under Stock Status, or the live page may show that the item is sold out. If you do run out of stock for an item, be sure to switch this to Out of Stock and it will remove that size/color variation from the live site.
Once you have entered the stock quantity in all the Variations you wish to edit, scroll down to the very bottom of the list and click the Save Changes button.