Letter replacement in string
<p>I have this simple code for letter substitution. What I would like to add is, that if i.e., I replace letter A with letter T, all T letters are automatically replaced with A as well. So if I have a word "atatatat", the following code changes the word to "tttttttt", but it should change it to "tatatata". How can I fix this?</p>

<pre><code>private void button3_Click(object sender, EventArgs e)
{
    String key= this.textBox1.Text;
    String letter1 = this.textBox2.Text;
    String letter2 = this.textBox3.Text;

    StringBuilder newKey = new StringBuilder();
    newKey.AppendLine(key);
    newKey.Replace(letter1, letter2);
    this.textBox4.Text = noviKljuc.ToString();
}
</code></pre>

<p>I tried with adding this line: <code>newKey.Replace(letter2, letter1);</code> But this changes word to "aaaaaaaa"</p>
