private void button1_Click(object sender, EventArgs e) {
  richTextBox1.SelectionFont = new Font(richTextBox1.Font,
    richTextBox1.SelectionFont.Style ^ FontStyle.Bold);
}

private void button2_Click(object sender, EventArgs e) {
  richTextBox1.SelectionFont = new Font(richTextBox1.Font,
    richTextBox1.SelectionFont.Style ^ FontStyle.Italic);
}

private void button2_Click(object sender, EventArgs e) {
    richTextBox1.SelectionFont = new Font(richTextBox1.Font,
      richTextBox1.SelectionFont.Style ^ FontStyle.Underline);