ye helpe kheyli kheli kochik :D

string strWord = "mehran";
string strQuiz = null;
for (int a = strWord.Length-1; a >= 0; a--)
strQuiz += strWord.Substring(a, 1);

        private void button3_Click(object sender, EventArgs e)
        {
            string strWord = this.textBox1.Text;
            string strQuiz = null;
            for (int a = strWord.Length - 1; a >= 0; a--)
              this.textBox2.Text= strQuiz += strWord.Substring(a, 1);
        }
    }
}