Variable wont change when I change it in script

Hi, so I just asked a previous question similar to this one, I'm sorry but not sure how to solve this one either. So I have an upgrade where the spawnTime decreases when you purchase it. Script :

     public Catch points;
 public SpawnItems spawnTime;
 public float cost;
 public int count;
 private float baseCost;
 public Text text;

 void Start ()
 {
     baseCost = cost;
 }

 public void PurchasedItem ()
 {
     if (points.points >= cost) {
         points.points -= cost;
         points.text.text = points.points.ToString () + " POINTS";
         spawnTime.spawnTime -= 4f;
         count += 1;
         cost = Mathf.Round (baseCost * Mathf.Pow (1.15f, count));
     }
         
 }

and the other SpawnItems script looks like so:

 public Transform[] SpawnPoints;
 public float spawnTime = 5f;

 public GameObject Collectables;

 void Start ()
 {
     InvokeRepeating ("SpawnCollectables", spawnTime, spawnTime);
 }

 void SpawnCollectables()
 {
     int spawnindex = Random.Range (0, SpawnPoints.Length);
     Instantiate (Collectables, SpawnPoints [spawnindex].position, SpawnPoints [spawnindex].rotation);
 }

So the problem is when I buy the upgrade that reduces the spawnTime does nothing, the reason why is because in the SpawnItems script I have the void Start () which is only run once and it does not update. So I need help on how to update the spawnTime in the SpawnItems script once you buy the upgrade. I tried void Update () but that ends up spawning a bunch of collectables.