[{"id":1,"product":[{"name":"Book","price":100},{"name":"Pen","price":10},{"name":"Book","price":100}]}]

db.collection.update({},[{$set:{p1:{$arrayElemAt:["$product",0]}}},{"$set":{"p1.paid":"$p1.price"}},{$project:{product:{$concatArrays:[["$p1"],{"$slice":["$product",1,{$size:"$product"}]}]}}}])