Slider.fillRect

Description

Optional RectTransform to use as fill 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 FillArea).
    public RectTransform newFillRect;

    //Deactivates the old FillRect and assigns a new one.
    void Start()
    {
        mainSlider.fillRect.gameObject.SetActive(false);
        mainSlider.fillRect = newFillRect;
    }
}