How to Change sprite for background by using the buttons

I have a dice game and in the main menu there are 4 buttons for the choice of background.
how can use the OnClick function and the Event Manager.
I don’t want to make new scene for every background.
What kind of script does it need ?

You just need a script that contains all the bacgrounds. Here’s my example:

public Image IMG_BG;
public Sprite[] Sprite_BG = new Sprite[4];
public void OnButtonClick(int index){
    	IMG_BG.sprite = Sprite_BG[index];
}