Change sprite collection

I need to change a sprite, but this is in another collection i found this:

var obj =Instantiate(prefab, new Vector3(0,0,-i),Quaternion.identity);
((Transform)obj).GetComponent<tk2dSprite> () = ("Collection2");

But it doesnt work u.u
im using :

((Transform)obj).GetComponent<tk2dSprite> ().SetSprite ("bg" + i);

For change the sprite and it works.

So, i just need to know how to change the sprite collection. Im using 2D toolkit Unity Ver 5.13 C#scripts

using UnityEngine;
using System.Collections;

public class MySprite : MonoBehaviour {

public tk2dSpriteCollectionData[] collections;//you'll need to import the collections you need directly from unity editor

// Use this for initialization
void Start () {
	GetComponent<tk2dSlicedSprite> ().Collection = collections[1];//index of collections array


//it worked for me