Setting GameObject in a script.

I have a projectile set as a game object, a 2d sprite prefab, I declared is at a public GameObject and was able to drag a prefab to make it work, no problem. In special circumstances I want it to be another prefab. How do I set the game object: Projectile to the special prefab.

thank you for all your help.

          switch (Convert.ToChar (valueOfTokens [1])) {
          case '+':
              genApples (float.Parse(numbers [temp]));
              break;
          case '-':
                               //set GameObject to new  prefab and generate some apples. 
              Projectile = GameObject.Find("Projectile_Prefab 1") as GameObject;
              genApples (float.Parse(numbers [temp]));
              break;
          default:
              genApples (float.Parse(numbers [temp]));
              break;
          }
      }
  void genApples(float apples){
      while (apples != 0) { //drop some  apples
          GameObject Goz = Instantiate (Projectile, ShootPt.transform.position, ShootPt.transform.rotation) as GameObject;
          apples--;
      }