Replace GameObject with different FBX

Hi all ! I got a little problem when replacing FBX, I want to replace my GameObject with different FBX in the same GameObject. here the code

 using UnityEngine;
 using System.Collections;
 
 public class MoveStage : MonoBehaviour
 {
     GameObject Pos1;
     GameObject Pos2;
     GameObject Pos3;
 
     GameObject Envi1;
     GameObject Envi2;
     GameObject Envi3;
 
     Vector3 PosEnd1;
     Vector3 PosEnd2;
     Vector3 PosEnd3;
 
     int LoopEnvi;
 
     void Start()
     {
         Pos1 = GameObject.CreatePrimitive(PrimitiveType.Cube);
         Pos1.transform.position = new Vector3(0, 0, -5.33f);
         Pos1.renderer.enabled = false;
         PosEnd1 = Pos1.transform.position;
 
         Pos2 = GameObject.CreatePrimitive(PrimitiveType.Cube);
         Pos2.transform.position = new Vector3(0, 0, 0f);
         Pos2.renderer.enabled = false;
         PosEnd2 = Pos2.transform.position;
 
         Pos3 = GameObject.CreatePrimitive(PrimitiveType.Cube);
         Pos3.transform.position = new Vector3(0, 0, 5.33f);
         Pos3.renderer.enabled = false;
         PosEnd3 = Pos3.transform.position;
 
         Envi1 = (GameObject)GameObject.Instantiate((GameObject)Resources.Load("FBX/Stage/komplek"));
         Envi1.transform.position = PosEnd1;
 
         Envi2 = (GameObject)GameObject.Instantiate((GameObject)Resources.Load("FBX/Stage/komplek"));
         Envi2.transform.position = PosEnd2;
 
         Envi3 = (GameObject)GameObject.Instantiate((GameObject)Resources.Load("FBX/Stage/komplek"));
         Envi3.transform.position = PosEnd3;
     }
 
     // Update is called once per frame
     void Update()
     {
         Envi1.transform.position += new Vector3(0, 0, 0.66f) * Time.deltaTime;
         Envi2.transform.position += new Vector3(0, 0, 0.66f) * Time.deltaTime;
         Envi3.transform.position += new Vector3(0, 0, 0.66f) * Time.deltaTime;
 
         if (Envi1.transform.position.z >= 10.66f)
         {
             Envi1.transform.position = PosEnd3;
             Envi2.transform.position = PosEnd1;
             Envi3.transform.position = PosEnd2;    
         }
         if (Envi2.transform.position.z >= 10.66f)
         {
             Envi2.transform.position = PosEnd3;
             Envi3.transform.position = PosEnd1;
             Envi1.transform.position = PosEnd2;
         }
         if (Envi3.transform.position.z >= 10.66f)
         {
             Envi3.transform.position = PosEnd3;
             Envi1.transform.position = PosEnd1;
             Envi2.transform.position = PosEnd2;
             LoopEnvi++;
         }
         Debug.Log(LoopEnvi);
         if (LoopEnvi > 2)
             //Here I want put code that replacing FBX in Envi1 GameObject with "FBX/Stage/pasar tradisional" in asset folder
     }
 }

how must I do? thanks for your attention