using System;

using System.Linq;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"

            {

              ""additionalFields"": [

                {

                  ""name"": ""NAME1"",

                  ""value"": ""VALUE1""

                },

                {

                  ""name"": ""NAME2"",

                  ""value"": ""VALUE2""

                },

                {

                  ""name"": ""NAME3"",

                  ""value"": ""VALUE3""

                },

                {

                  ""name"": ""NAME4"",

                  ""value"": ""VALUE4""

                }

              ]

            }";

​

        var rootObject = JToken.Parse(json);

​

        // Get a reference to the array of objects as before

        var additionalFields = rootObject["additionalFields"];

​

        // Find the object we need to change in the array

        var additionalField = additionalFields.FirstOrDefault(f => string.Equals((string)f["name"], "NAME1", StringComparison.CurrentCultureIgnoreCase));

​

        // if the object is found, update its properties

        if (additionalField != null)

        {

            additionalField["name"] = "newname";

            additionalField["value"] = "newvalue";

        }

        Console.WriteLine(rootObject.ToString());

    }

}

​