db={"sample":[{current_book:"Aurthor",upcomming_book:["Sigma","Rocky"]}]}

db.sample.update({"upcomming_book.0":{$exists:true}},[{"$set":{current_book:{$arrayElemAt:["$upcomming_book",0]},"upcomming_book":{$slice:["$upcomming_book",-1]}}}])