Database
[{arr:[{"coursesMarks":{"_id":"634a9be567a1f07be02f71d8","courseCode":"cse1201","courseTitle":"SP"}},{"coursesMarks":{"_id":"634a9be567a1f07be02f71db","courseCode":"cse1203","courseTitle":"DS"}}]}]
Query
db.collection.update({},[{$addFields:{courses:{$map:{input:"$arr",in:"$$this.coursesMarks"}}}},{$unset:"arr"}])