Mouse flying

Hello. I'm new to coding, but, i found some codes on internet, you can fly with mouse, but there isn't any code that allow you to fly upwards or downwards with mouse... My actual code is there:

 var rotateSpeed = 25.0;
 var speed = 50.0;
 function Update() {
 var transAmount = speed * Time.deltaTime;
 var rotateAmount = rotateSpeed * Time.deltaTime;
 
 if (Input.GetKey("w")) {
 transform.Rotate(rotateAmount, 0, 0);
 
 }
 if (Input.GetKey("s")) {
 transform.Rotate(-rotateAmount, 0, 0);
 }
 if (Input.GetKey("a")) {
 transform.Rotate(0, -rotateAmount, 0);
        var speed = 0;
 }
 if (Input.GetKey("d")) {
 transform.Rotate(0, rotateAmount, 0);
 }
 
  }

You can rotate by a and d, go backwards by s and forward by w, but if i look up or down, it don't do anything... Can you please help me? I know it is now overlooked, but i'm new to forum too :). So if there is good soul, that help me, please respond. Thank you. And sorry for my english, i'm not from England or America.

MinoMMN.