[{item:"journal",instock:[{warehouse:"A",qty:5},{warehouse:"C",qty:15}]},{item:"notebook",instock:[{warehouse:"C",qty:5}]},{item:"paper",instock:[{warehouse:"A",qty:60},{warehouse:"B",qty:15}]}]

db.collection.update({"instock.warehouse":"C","item":"journal"},{$set:{"instock.$.qty":10}})