On the menu, access Products

Tap on  Stock and then on a product.

Activate Stock Management  for this product.

Tap to enter your stock.

Enter the stock, and then tap Ok.

You can also select a minimum stock, to know when this product is about to end, and view all the movements of this product.

On the checkout the products below minimum are displayed with an oragne circle on the upper right corner. A red circle means that the product is out of stock.

If you long press a product you’ll be able to see the available stock.