using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;
public class ChangeMat : MonoBehaviour {


    [MenuItem("Tools/ChangeNewMat")]
    static void ChangeNewMat()
    {
        GameObject obj = Selection.activeGameObject;
        foreach (Renderer it in obj.transform.GetComponentsInChildren<Renderer>())
        {
            Material mat = AssetDatabase.LoadAssetAtPath<Material>("Assets/ViveJx/Materials/JianXiuBall.mat"
 
);
            Material[] bufMat = new Material[it.sharedMaterials.Length];
            for (int i = 0; i < it.sharedMaterials.Length; i++)
            {
                bufMat[i] = mat;
            }
            it.sharedMaterials = bufMat;
        }
    }
}

Note that like all arrays returned by Unity, this returns a copy of materials array. If you want to change some materials in it, get the value, change an entry and set materials back.
要改變材質陣列，得重新賦值一個數組