How to Change Background Sprite at run time ?

I am making a game having different scenarios. It is an endless runner with four different characters . After certain score amount I want to change the sprite in background and also the character itself .I have written a code to spawn background one after other .

using UnityEngine; using System.Collections;

public class SpawnerScript1 : MonoBehaviour {

 public  GameObject background;
 public Transform bga;
 public Transform bgb;
 float finalpos;
 public float spawnTime;
         
 // Use this for initialization
 void Start () {
     Spawn ();
     finalpos = bgb.position.x - bga.position.x;

     }

 void Spawn()
 {
     bgb.position = new Vector3 (bgb.position.x + finalpos, bgb.position.y , bgb.position.z);
     Instantiate (background, b.position, Quaternion.identity);
     Invoke ("Spawn", spawnTime);
 }

 public void Stopinvokation()
 {
     CancelInvoke ("Spawn");
 }

}

Now , How can I change background after some time .So that this same code snippet is used but with other background on it .Just Keep in mind that I have to change the sprite from other script not this one , from the script in which i am calculating the score .