how to delet from textbox ?

private void myTextBox_KeyPress(object sender, KeyPressEventArgs e)
    {
    TextBox tb = sender as TextBox;
    if (tb != null)
        {
        string existing = tb.Text;
        if (existing.Length >= 7)
            {
            tb.Text = existing.Substring(existing.Length - 6);
            tb.SelectionStart = tb.Text.Length;
            }
        }
    }

if (textBox1.Text.Length > 7)
{
   textBox1.Text = textBox1.Text.Remove(0, 7);
   textBox1.SelectionStart = textBox1.Text.Length;
}

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            string FirstSeven;
            if (textBox1.Text.Length >= 7)
            {
                FirstSeven = textBox1.Text.Substring(0, 7);
                textBox1.Text = textBox1.Text.Substring(7);
            }
         }