using System;

public class Program

{

    public static void Main()

    {

        string testString = "Hello, I am Hello, You are Declan";

        Console.WriteLine(Replace(testString, "Hello", "Mark", 2));

    }

        public string Replace(string text, string search, string replace, int instanceNumber)

        {

          int pos = 0;

          for(int i=1; i<instanceNumber;i++)

          {

            pos = text.IndexOf(search, pos);

          }

​

          if (pos < 0)

              throw new Exception("There were only " + instanceNumber + " instances of the phrase");    

          return text.Substring(0, pos) + replace + text.Substring(pos + search.Length);

        }

}