добавить скрипт к кубу, примерно так:
void Update()
{
if(Input.touches > 0)
{
Touch touch = Input.GetTouch(0);
Vector2 pos = touch.position;
float width_fraction = pos.x / (float)Screen.width;
// only run this, if finger is on the right half (50%)
if(width_fraction > 0.5)
{
this.transform.Rotate(0, Time.deltatime * 30f, 0); // rotate 30 Deg per second
}
}
}