[{"_id":1,"subject":[{"subjectId":1,"flashcard":[{"_id":1,"title":"Card 1","tag":[]},{"_id":2,"title":"Card 2","tag":[]}]}]}]

db.collection.update({_id:1/**classroomId,*/},{"$set":{"subject.$[subject].flashcard.$[flashcard].title":"new title"}},{arrayFilters:[{"subject.subjectId":1/**subjectId*/},{"flashcard._id":1/**flashcardId*/}]})