Reward System Logic Problem

Guys hi. I Tried a making 3 star reward system and ı did it basicly. The system working normaly if ı pass the level under 7 step im earning 3 star. (int step <7 ) 3 Star (int step >7 and <12) 2 star (int step >12) 1 star so, here is the problem. For example ım earned 3 star in lvl 1. Later i played the lvl 1 again and ı pass the "level 1" >12 step. Code is change the stars to 1 stars. I want if ı earned before 3 star dont show 2 or 1 star. If ı earned 2 stars before dont show 1 star.

My c# skill not enough for writing to code this status.

     LEVELS Class
 public Sprite Star1;
 public Sprite Star2;
 public Sprite Star3;

 public Image[] Stars;

    void Start () {
 
     //PlayerPrefs.SetInt("Level 1",1);

         if (PlayerPrefs.GetInt("Level 1") ==1){

         
         LevelNumbers[0].text = "1";
         PlayerPrefs.SetInt("Level 1",1);

         if (PlayerPrefs.GetInt("L1St")==1){
         Stars[0].sprite = Star1;
     
         
         }
         else if (PlayerPrefs.GetInt("L1St")==2){
             Stars[0].sprite = Star2;
         }

     else if (PlayerPrefs.GetInt("L1St")==3){
             Stars[0].sprite = Star3;
         }
     
     }

        void OnCollisionEnter(Collision coll) {
     if (man1 == false & man2 == false &(coll.gameObject.tag=="Finish")) {
     //    Application.LoadLevel("Levels");

         PlayerPrefs.SetInt("Level 1",1);

         if(Application.loadedLevelName == "Level1"){

             PlayerPrefs.SetInt("Level 2",1);
     
             if(step >12 )
             {
                 if (PlayerPrefs.GetInt("L1St") ==0){  
                 PlayerPrefs.SetInt("L1St",1); /// Do 1 Star
                 

                 }
             }



             if(step >7 && step <12 )
             { 
                 if (PlayerPrefs.GetInt("L1St") ==0 || PlayerPrefs.GetInt("L1St")==1  )
                 {
                 PlayerPrefs.SetInt("L1St",2);  /// Do 2 Star
                     
                 
                 }
             
             
             }


             if(step <=7 )
             {
                 if (PlayerPrefs.GetInt("L1St") ==0||PlayerPrefs.GetInt("L1St") ==1 || PlayerPrefs.GetInt("L1St") ==2 ) 
                  {
                 PlayerPrefs.SetInt("L1St",3); /// Do 3 Star
                     
                 }
             }

             Application.LoadLevel("levels");

         }