Database
[{"_id":ObjectId("575845a713d284da0ac2ee81"),"Profession_id":"575841b313d284da0ac2ee7d","Prof_Name":"Chief Officer"},{"_id":ObjectId("575845d213d284da0ac2ee82"),"Profession_id":"575841b313d284da0ac2ee7d","Prof_Name":"Executive Officer"}]
Query
db.collection.update({},[{$set:{"Profession_id":{$toObjectId:"$Profession_id"}}}],{"multi":true})