[{"username":"abc","notes":[{"topic":"hello","content":"world"},{"topic":"bye","content":"universe"},{"topic":"xxxxxxx","content":"xx"}],"__v":31}]

db.collection.update({"username":"Dhruv70"},[{"$set":{"notes":{$concatArrays:[{"$ifNull":["$notes",[]]},[/** your note entry to be upserted here*/{"topic":"new note","content":"to insert"}]]}/** other fields that you want to set here*/}}],{upsert:true})