[{"key":1,"lastViewed":[{"title":"n1","page":1},{"title":"n3","page":1},{"title":"n4","page":1}]},{"key":2,"lastViewed":[{"title":"n2","page":2}]}]

db.collection.update({"key":1,"lastViewed.title":"n3"},{$set:{"lastViewed.$.page":4}})