Hai guys i need help why my scripts not running "i will make button next3dobject and prev3dobject"

Hi, you could use a property to swap objects instead of doing it in Update.

Here, some code with comments :

 public class ObjectListManager : MonoBehaviour
 {
     [SerializeField] List<GameObject> objectList; // use SerializeField attribute to display/save in the editor, while not accessible from other classes
 
     private int _current = 0;
     private int Current
     {
         get {return _current;}
         set
         {
             if (value != _current) // if value is different
             {
                 if (value >= 0 && value < objectList.Count); // if value is within List
                 {
                     objectList[_current].SetActive (false); // disable previous
                     _current = value;
                     objectList[_current].SetActive (true); // enable new
                 }
             }
         }
     }
 
     void Awake ()
     {
         // Disable all but first object
         for (int i = 1 ; i < objectList.Count ; i++)
             objectList[i].SetActive (false);
     }
     public void Next ()
     {
         // cycle when Current reaches objectList.Count-1
         Current = Current < (objectList.Count -1) ? Current+1 : 0;
     }
     public void Previous ()
     {
         // cycle when Current reaches 0
         Current = Current > 0 ? Current-1 : objectList.Count-1;
     }
 }