using System;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"

            {

              ""payload"": [

                {

                  ""a"": ""yes"",

                  ""b"": ""no"",

                  ""c"": ""maybe""

                },

                {

                  ""a1"": ""agg"",

                  ""b"": ""no"",

                  ""c"": ""maybe""

                },

                {

                  ""a"": ""L"",

                  ""b"": ""k"",

                  ""c"": ""maybe""

                }

              ]

            }";

        JToken root = JToken.Parse(json);

        foreach (JToken match in root.SelectTokens("payload[*].a"))

        {

            match.Replace(new JValue("ZZZZZZZZ"));

        }       

        Console.WriteLine(root.ToString());

    }

}

​

​