if i understood correctly, you need a code example for the turning left/right with keyboard input instead of mouse?
in that case:
if (Input.GetKey(KeyCode.LeftArrow))
{
rb2d.AddTorque(torqueAmount * Time.deltaTime);
}
else if (Input.GetKey(KeyCode.RightArrow))
{
rb2d.AddTorque(-torqueAmount * Time.deltaTime);
}