Database
[{"genres":"[{\"id\":16,\"name\":\"Animation\"},{\"id\":35,\"name\":\"Comedy\"},{\"id\":10751,\"name\":\"Family\"}]"}]
Query
db.collection.aggregate([{"$project":{"genres":{"$function":{"body":"function(genres) { return JSON.parse(genres) }","args":["$genres"],"lang":"js"}}}},{"$merge":{"into":"collectionName","on":"_id","whenMatched":"replace","whenNotMatched":"insert"}}])