How to rotate a 2d object with joystick in unity ?

In desktop version, my character's arm is rotates according to the movement of the mouse. I need the same thing in mobile version but this time I need a joystick to rotate his arm. How can I do that?

  //Here is my code, for rotating arm
  void Update()
 {
     Vector3 difference = Camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position; 
     difference.Normalize();
     float rotZ = Mathf.Atan2(difference.y, difference.x) * Mathf.Rad2Deg; 
     transform.rotation = Quaternion.Euler(0f, 0f, rotZ); 
 }