[{"userId":"10000001","name":"Josh","booksBought":[{"booktitle":"harry potter","quantity":1},{"booktitle":"game of thrones","quantity":2}]},{"userId":"10000002","name":"Carl","booksBought":[{"booktitle":"lord of the rings","quantity":4}]}]

db.collection.update({"userId":{$in:["10000001","10000002"]}},{$inc:{"booksBought.$[elem].quantity":10}},{arrayFilters:[{"elem.booktitle":"harry potter"}]})