How to make object move and rotate

I'm New in unity3d and game development also. i just start to make game runnig style like temmple run. i add a cube in scene and assign this script for movement

 function Update () {
 transform.Translate (transform.forward * 2.0f * Time.deltaTime);
 if (Input.GetKey ("up")) {
  transform.Rotate (Vector3.up * 90.0f  ); ;
 }
 }

when i play cube moving correctly. but when press up button the cube not rotate 90 degree but just move forward and backward.