[{_id:ObjectId("609391f436e519039a634311"),name:"Class A",numOfStudents:10,students:[{name:"Student A",age:10,},{name:"Student B",age:10,}]}]

db.collection.update({"_id":ObjectId("609391f436e519039a634311")},[{"$set":{"something":"$name","students":{"$map":{"input":"$students","in":{"name":"$$this.name"}}}}}])