Changing a button texture in Selection Grid

Hi all! I have a bunch of buttons arranged in a selection grid. Can someone please tell me how I can access each of these buttons individually to change their textures?

public int selected = 0;
public int columns = 3;
public Texture images;

void OnGUI(){
	selected = GUI.SelectionGrid(new Rect (400,200,300,100), selected, images, columns);

hope tht help