Set text label, next sceen, read json with Unity3D

using UnityEngine;
using System.Collections;

public class nextScreen : MonoBehaviour {

    // Use this for initialization
    public void chuyenManHinh(){
        Debug.Log ("Thieu mao");
        //Application.LoadLevel (1);
        UILabel subtitle = GameObject.Find("LabelHienThi").GetComponent<UILabel>();
        subtitle.text = "Toan dep trai";
    }

    IEnumerator Start()
    {
        //Load JSON data from a URL
        string url = "http://paultondeur.com/files/2010/UnityExternalJSONXML/books.json";
        url = "https://dl.dropboxusercontent.com/u/119735704/API/store/link.txt";
        WWW www = new WWW(url);

        //Load the data and yield (wait) till it's ready before we continue executing the rest of this method.
        yield return www;
        if (www.error == null)
        {
            //Sucessfully loaded the JSON string
            Debug.Log("Chuoi json: " + www.text);

            //Process books found in JSON file
            //ProcessBooks(www.text);
        }
        else
        {
            Debug.Log("Loi: " + www.error);
        }

    }
}