If you are talking about Product's quantity on hand, it can be retrieved from qty_available (Quantity On Hand) field of the Product.  It is a calculated field.  To update, you cannot directly update the qty_available field, because it is a calculated field.  You need to actually create a stock.move to account for the quantity difference.  One mechansim that Odoo has provided for your is through Physical Inventory.  Another way it from the Update button next to the qty_available field which in turn will call the stock.change.product.qty wizard (odoo/addons/stock/wizard/stock_change_product_qty.py) that will create the new stock.move.