Manipulating Keyboard Input

private void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            Keys k = (Keys)sender;
            if(k.GetType = Keys.A)
            e.Handled = false; // to check it to fulfill
            
            // Do as your wish
        }

private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyData.ToString().Equals("A"))
    {
        textBox1.Text += "B";
    }
}

MyTextBox.KeyPress += (sender, eventArgs) => {
    if (eventArgs.KeyChar == 'A')
        eventArgs.KeyChar = 'B';
};