using System;

using System.Linq;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        // Load the JSON from a file into a JObject

        JObject o1 = JObject.Parse(File.ReadAllText(@"output.json"));

​

        // Get the desired property whose value is to be replaced

        var prop = o1.Property("to_Description");

​

        // Replace the property value with the first child JObject of the existing value

        prop.Value = prop.Value.Children<JObject>().FirstOrDefault();

​

        // write the changed JSON back to the original file

        File.WriteAllText(@"output.json", o1.ToString());       

    }

}

​

​

// Simulate the System.IO.File class, since we can't read/write real files on .Net Fiddle

public static class File

{

    public static string ReadAllText(string filename)

    {

        return @"

            {

              ""Product"": ""123"",

              ""to_Description"": [

                {

                  ""ProductDescription"": ""Product 1""

                }

              ]

            }";

    }

    public static void WriteAllText(string filename, string contents)

    {

        Console.WriteLine(contents);

    }

}