using System;

using Newtonsoft.Json.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var json = "{\"title\" : { \"newTitle\" : \"Test\"}, \"tags\" : { \"newTags\" : [\"Tag1\", \"Tag2\"] }}";

​

        var title = "Test2";  // "Test2" in title

        List<string> tags = new List<string> {"TagA", "TagB", "TagC"}; // "TagA", "TagB", "TagC" in tags

​

        dynamic root = JObject.Parse(json);

        JObject titleObj = (JObject)root["title"];

        titleObj["newTitle"] = title;

        JObject tagsObj = (JObject)root["tags"];

        tagsObj["newTags"] = JArray.FromObject(tags);

        Console.WriteLine(root);

    }

}