public int rotationOffset = 90;

void Update () {
    Vector3 difference = Camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position; // subtracting the position of the player from the mouse position.
    difference.Normalize(); //normalizing the vector meaning all the sum of the vector will be = to 1.

    float rotZ = Mathf.Atan2(difference.y, difference.x) * Mathf.Rad2Deg;//find the angle in degree.
    transform.rotation = Quaternion.Euler(0f, 0f, rotZ + rotationOffset);

}