[{_id:1,"name":"Sherlock Holmes","address":{"house":"221b","street":"Baker St","city":"London",}},{_id:2,"name":"James Bond","address":{"door_number":"30","address1":"Wellington Square","city":"London",}}]

db.collection.update({},[{"$addFields":{"merged":{"$mergeObjects":[{/** fields that you want to keep in outer*/_id:"$_id",name:"$name"},"$address"]}}},{"$replaceRoot":{"newRoot":"$merged"}}],{multi:true})