using System;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string jsonStr = "{\"Root\":[{\"Item_Name\":\"Test\",\"Items\":[{\"Item_id\":\"1\",\"revs\":[{\"rev_id\":\"1\"},{\"rev_id\":\"3\"}]},{\"Item_id\":\"2\",\"revs\":[{\"rev_id\":\"1\"}]}]}]}";

        JObject jsonObject = JObject.Parse(jsonStr);

        JObject new_rev = new JObject();

        new_rev["rev_id"] = "need to add new entry here";

        JArray items = jsonObject["Root"][0]["Items"].Value<JArray>();

        foreach(var item in items){

            if(item.Value<JObject>()["Item_id"].Value<string>() == "1"){

                item["revs"].Value<JArray>().Add(new_rev);

                break;

            }

        }

        Console.WriteLine(jsonObject.ToString());

    }

}