mouse control with slower rotation

hi, i need help with completion of my player control script. i try change rotation control from keycode to mouse but now i dont know how make rotation slowly. here is my script:

 #pragma strict
 
 var speed = 0f;
 var score = 0;
 var maxspeed : float;
 var acceleration : float;
 var SpawnPos : Vector3;
 static var Plife = 3f;
 var animator: Animator;
 var mousepos : Vector3;
 
 function Start () {    
 }
 
 function Update () {
 
     var objectPos = Camera.main.WorldToScreenPoint(transform.position);
     var mousepos = Input.mousePosition - objectPos; 
     transform.rotation = Quaternion.Euler (Vector3(0,0,Mathf.Atan2 (mousepos.y,mousepos.x)* Mathf.Rad2Deg)); 
     transform.Rotate(Vector3(0,0,-90));    //i need to make this rotate slower.
     
     if(Plife <= 0f){
         transform.position = SpawnPos;
         Plife = 3f;
         score -= 1;
         speed = 0f;
         NewPlShooting.spawnDelay = 0.1f;
     }
     
     GUILayerScript.speed = speed;
     GUILayerScript.life = Plife;
     GUILayerScript.Pscore = score;
     GUILayerScript.position = transform.position;
     NewPlShooting.Pspeed = speed / 40;
     ParticleControler.speed = speed;
     
     if(Input.GetKey(KeyCode.C)) {
         if(GetComponent(CircleCollider2D).enabled == false) {
             GetComponent(CircleCollider2D).enabled = true;
         }
         else if(GetComponent(CircleCollider2D).enabled == true) {
             GetComponent(CircleCollider2D).enabled = false;
         }
     }
 }
 
 function FixedUpdate() {
     if(Input.GetKey(KeyCode.W)) {
         rigidbody2D.velocity = transform.up * speed * Time.deltaTime;
         animator.SetBool("stop", false);
         animator.SetBool("stop2", false);
         if(speed < maxspeed) {
             speed += acceleration;
         }
     }    
     else if(Input.GetKey(KeyCode.S)) {
         rigidbody2D.velocity = transform.up * speed * Time.deltaTime;
         if(speed > -400) {
             speed -= acceleration * 2;
         }
     }
     else {        
         rigidbody2D.velocity = transform.up * speed * Time.deltaTime;
         animator.SetBool("stop", true);
         animator.SetBool("stop2", true);
         if(speed > 0) {    
             speed -= 1f;
         }
         else if(speed < 0) {
             speed += 1f;
         }
     }
 }
 
 function OnCollisionEnter2D(col : Collision2D){
     if (col.gameObject.tag == "bullet1") {    
         Plife -= 1f;
     }
 }

thx for help.