Instantiating gameObjects and prefabs issues.

I want to make it so when a button is clicked the gameObject saved to a variable changes and instantiates a different prefab. However the prefab that is loaded seems to be dependent on what is assigned to the script in the inspector. How can I change what is assigned to it from code?

This is what I have so far:

 var tile = (Resources.Load("prefabs/up"));
 
 function OnMouseOver(){
 var tile = (Resources.Load("prefabs/up"));
 if(Input.GetMouseButtonDown(0)){
 if(gameObject.transform.tag == "tile"){
 var p = Vector3(transform.position.x, transform.position.y - 0.4, 0);
 Instantiate(tile, p, Quaternion.Euler(0, 0, 90));
 destroy.money -= 50;
 }
 }
 
 }

So when I click a button I want the tile variable to change to a different prefab. Resources.Load does not seem to work and as I said earlier, the prefab that load appears to be dependent on what is selected from the inspector.