All inventory management can be done through the Products menu in your app. Click the Stock tab to see the amount available on each product, as well as a summary of the total projected cost and profit of your inventory. See more details using the filter function at the top right of the screen.
You can turn on stock management from within each product's details, just select the desired item, tap on the Stock tab and activate the Manage inventory for this product. Take the opportunity to add the amount of items you have on hand and set a minimum amount for the product to appear with an orange circle in the Checkout menu.