Transform.SetAsFirstSibling

Description

Move the transform to the start of the local transform list.

using UnityEngine;
using System.Collections;
using UnityEngine.UI; //Required when using UI Elements.
using UnityEngine.EventSystems; // Required when using event data.

public class ExampleClass : MonoBehaviour, IPointerDownHandler
{
    public RectTransform panelRectTransform;

    //Invoked when the mouse pointer goes down on a UI element.
    public void OnPointerDown(PointerEventData data)
    {
        // Puts the panel to the back as it is now the first UI element to be drawn.
        panelRectTransform.SetAsFirstSibling();
    }
}