using System;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        String s = "thaaaa"; //I want to replace h with a and vice versa

        s = ReplaceChar(s, 'a', 'd', 5);

        Console.WriteLine(s);

    }

​

    public static string ReplaceChar(string s, char c, char d, uint index)

    {

        if(index > s.Length)

            return s;

        int count = 0;

        StringBuilder b = new StringBuilder();

        foreach (char a in s.ToCharArray())

        {

            if (a == c)

            {

                count++;

                if (index == count)

                {

                    b.Append(d);

                    continue;

                }

            }

​

            b.Append(a);

        }

​

        return b.ToString();

    }

}