How to Change a prefab with Code

Think of prefab as a template for an object. On these templates, you can create specific behaviours that will do whatever you tell the template to do. Behaviours are just scripts, that define how the object works. So to change the prefab's values, you need to add a script to it, and inside of the script define the values that you want to be changed and assign to them the specific components. Then within another script that you want those values changing, you get that prefab and access it's behaviour.

There is a simple tutorial here:

https://docs.unity3d.com/Manual/ControllingGameObjectsComponents.html

From where you want to change it? From shop or In-game while playing? You want to have different buttons to assign that gun or you want to assign them by scrolling.

Still, I would like to give you some idea how you can do it.

     public GameObject[] weaponPrefab;
     private int weaponNo;
     
     void Start()
     {
     weaponNo = 0;
     }
     void Update()
     {
     if(weaponNo == 0)
     {
     weaponPrefab[0].setActive(true);
     weaponPrefab[1].setActive(false);
     }
     else if(weaponNo == 1)
     {
     weaponPrefab[0].setActive(false);
     weaponPrefab[1].setActive(true);
     }
     }