OnMouseDown() help

how to use OnMouseDown() instead of if(Input.GetKeyDown(“s”))?
here is my code…

using UnityEngine;
using System.Collections;

public class shuffle : MonoBehaviour {
	float pos1_x=-3.5f;
	float pos1_y=-3.5f;
	float pos2_x=6.0f;
	float pos2_y=2.5f;
	float pos3_x=-3.5f;
	float pos3_y=3.5f;
	float pos4_x=-6.0f;
	float pos4_y=2.5f;
	int[] cardno=new int[32];

	
	void  Update (){		
		Transform[] allChildren= gameObject.GetComponentsInChildren<Transform>();
		int i=-1;
		int j;
		foreach(Transform child in allChildren) {      
			if(child.gameObject.name.Length<4)
			{
				i++;
				cardno[i]= child.gameObject.GetComponent<SpriteRenderer>().sortingOrder;  
			} 
		}		
		if(Input.GetKeyDown("s")){		
			for(int m=0;m<9;m++){ 
				card_Shuffle(cardno);
				i=-1;
				foreach(Transform child in allChildren) {      
					if(child.gameObject.name.Length<4)
					{
						i++;
						child.gameObject.GetComponent<SpriteRenderer>().sortingOrder=cardno[i];  
					} 
				}
			}
			i=-1;
			j=0;
			foreach(Transform child in allChildren) {      
				if(child.gameObject.name.Length<4)
				{
					i++;
					child.gameObject.GetComponent<SpriteRenderer>().sortingOrder=cardno[i];
					if(cardno[i]>=0  cardno[i]<4){
						child.gameObject.transform.position=new Vector3(pos1_x+1f*j,pos1_y,0);
						child.gameObject.GetComponent<SpriteRenderer>().sortingOrder=j;
						j++;
					}
					else if(cardno[i]>=4  cardno[i]<8) child.gameObject.transform.position=new Vector3(pos2_x,pos2_y-.75f*(cardno[i]-4),0);
					else if(cardno[i]>=8  cardno[i]<12) child.gameObject.transform.position=new Vector3(pos3_x+1f*(cardno[i]-8),pos3_y,0);
					else if(cardno[i]>=12  cardno[i]<16) child.gameObject.transform.position=new Vector3(pos4_x,pos4_y-.75f*(cardno[i]-12),0);
				}
			}
		}
        }

OnMouseDown() is a message which is passed to your object (like Update()).

I think you are looking for this: