How Increase And Decrease And Reset Spawn Rate Over

Hi How I Can Increase And Decrease Spawn Rate Every Time I Want? My Code Work Just For Increase Not Decrease Or Reset!

public float nextDrop = 0f; public float dropInterval = 2f; public float changeInterval = 5f; float delay = 3f; int minX = -6; int maxX = 6; int direction; int indexx; public Vector3 spawnvalue; public Transform fireworkY; public GameObject explosion; public GameObject speedparticla; public GameObject yourloseTable; public GameObject TestEffect; public GameObject[] targets; public Canvas touch; bool gameOver = false; public static bool SpawnGo; public Text theyarecoming; public Text theyarecoming2; public Text ScoreText; public ScoreManager thescore; public AudioClip deathsound; public AudioClip EndGame; public AudioClip FireWorks;

 void Start()
 {
     

     ScoreText.enabled = true;
     SpawnGo = true;
     WaterMove.OceanMove = true;
     yourloseTable.SetActive(false);
     touch.gameObject.SetActive(true);
     nextDrop = Time.time + delay;
     Pool.index = 0;
     Invoke ("Coin", (Random.Range(10f, 50f)));
 }
 
 void Update()

{ if(PlayerSelectionGame.index == 0) indexx = 0;

     if(PlayerSelectionGame.index == 1)
     indexx = 1;

     if(PlayerSelectionGame.index == 2)
     indexx = 2;

 if(SpawnGo)
 {

     if(Time.time >= nextDrop)
     {
           Spawn();

           nextDrop += dropInterval;
           if(Time.timeSinceLevelLoad >= changeInterval) 
          {
            if(dropInterval > 0.3f)
            {
              dropInterval *= 0.92f;
            }
          
          }
           if(Time.timeSinceLevelLoad >= 50f)
          {
            if(dropInterval < 0.3f)
            {
              dropInterval = 2f; 
            }
          }
           if(Time.timeSinceLevelLoad >= 60f)
          {
            if(dropInterval > 0.3f)
            {
              dropInterval *= 0.92f;
            }
           
          }
           if(Time.timeSinceLevelLoad >= 100f) 
          {
            Pool.index = 1;
            ShowTextAnimation();
            Debug.Log("startwave");
          }
           if(Time.timeSinceLevelLoad >= 120)
          {
            Pool.index = 2;
            Debug.Log("stoptwave");
          }
           if(Time.timeSinceLevelLoad >= 160)
          {
            ShowTextAnimation2();
          }
     }
 }

}

 void Spawn()
 {
     GameObject spawn = Pool.Instance.GetPool();
     spawn.transform.position = new Vector3 (Random.Range (minX, maxX), spawnvalue.y, targets[indexx].transform.position.z + 50 );
     spawn.SetActive(true);
 }