db={"Employee":[{"_id":1,"name":"almonds","age":120},{"_id":2,"name":"bread","age":80},{"_id":3,"name":"cashews","age":60},{"_id":4,"name":"pecans","age":70},{"_id":5,"name":"test","age":92},]}

db.Employee.aggregate([{$sort:{instock:-1}},{$limit:2},{$set:{Benefits:["Vacation","Rolex"]}},{$merge:{into:"Employee",on:"_id",/** Optional*/whenMatched:"replace",/** Optional*/}}])