Database
[{"_id":{"$oid":"000000000000000000000001"},"side1":[{"username":"test1","id":"id1",},{"username":"test2","id":"id2",},{"username":"test3","id":"id3",}],"side2":[{"username":"test4","id":"id4",},{"username":"test5","id":"id5",},{"username":"test6","id":"id6",}],}]
Query
db.collection.update({"_id":ObjectId("000000000000000000000001"),"side1.username":"test1"},{"$set":{"side1.$.id":"43242342"}})