Slider.handleRect

Description

Optional RectTransform to use as a handle for the slider.

using UnityEngine;
using System.Collections;
using UnityEngine.UI; // Required when Using UI elements.

public class Example : MonoBehaviour
{
    public Slider mainSlider;
    //Reference to new "RectTransform" (Child of "Handle Slide Area").
    public RectTransform handleHighlighted;

    //Deactivates the old Handle, then assigns and enables the new one.
    void Start()
    {
        mainSlider.handleRect.gameObject.SetActive(false);
        mainSlider.handleRect = handleHighlighted;
        mainSlider.handleRect.gameObject.SetActive(true);
    }
}