How to GUI texture image change?

GUI Button in texture image mapping,
on button clicked, want texture image change…

how to GUi texture image change?

Dude, changing the texture of a guitexture is a simple process, you can do it either with a GUI Skin as suggested by PushpaK , or check this example… it might help you…
just comment if anything is not clear about the code

public var yourGUITexture:GUITexture;
public var texture1:Texture2D;
public var texture2:Texture2D;
private var yourCurrentTexture:Texture2D;
private var showAlert = false;

// whenever you will show the window just set showAlert = true;

function OnGUI()
{
	if(showAlert)	
	   var window = GUI.Window(0 , Rect(posX,posY,someWidth,someHeight), ShowAlert ,"");

    yourGUITexture.texture = yourCurrentTexture;
}

function ShowAlert(windowID : int)
{
     if (GUI.Button (Rect(posX,posY,someWidth,someHeight), "LeftButton")) 
{	  
    showAlert= false;
    yourCurrentTexture = texture1;
} 

    if (GUI.Button (Rect(posX*2,posY,someWidth,someHeight), "RightButton")) 
{	  
    showAlert= false;
    yourCurrentTexture = texture2;
} 

}