Change Texture

  Hello Unity3D.I have a problem with Texture2D.My problem is that i want to change the texture2D every 5 seconds but i can't seem to make it happen.For Example.I am making a countdown for my game and i need the texture to change so that the players know when to fight.If anyone knows how i can make it that i can change my texture2D every 5 seconds.Can you please tell me how

Script

 var AboutToBeLit : Texture;
 var Round1 : Texture;
 var LetsGetIt    : Texture;
 var CharacterName : Texture;
 var AboutToBeLitLength : float;
 var Round1Length : float;
 var x : float;
 var XAboutToBeLit : float;
 var YAboutToBeLit : float;
 var XRound1: float;
 var YRound1: float;
 var XLetsGetIt : float;
 var YLetsGetIt : float;
 var AboutToBeLitwidth: float;
 var Round1width: float;
 var nameWidth: float;
 var nameLength: float;
 var y : float; 
 var LetsGetItLength : float; 
 var LetsGetItWidth : float;
 var Xname : float;
 var Yname : float;
 var GetLit : AudioClip;
 var Round : AudioClip;
 var GetIt : AudioClip;
 var GUIEnabled : boolean = false;
  
 function OnGUI() {
 
 
 
 GUI.DrawTexture(Rect((Screen.width/2) - XAboutToBeLit, YAboutToBeLit, AboutToBeLitLength, AboutToBeLitwidth), AboutToBeLit);{
 AboutToBeLit.rend.material.mainTexture = AboutToBeLit;  
 yield WaitForSeconds(1);
 GUI.DrawTexture(Rect((Screen.width/2) - XRound1, YRound1, Round1Length, Round1width), Round1); 
 yield WaitForSeconds(1);
 GUI.DrawTexture(Rect((Screen.width/2) - x, y, LetsGetItLength, LetsGetItWidth), LetsGetIt);
     }
 }
  
 function Start () {
 rend = GetComponent.<Renderer>();
 GUI.DrawTexture(Rect((Screen.width/2) - XAboutToBeLit, YAboutToBeLit, AboutToBeLitLength, AboutToBeLitwidth), AboutToBeLit);
 AboutToBeLit.rend.material.mainTexture = AboutToBeLit;  
 yield WaitForSeconds(5);
 Round1.rend.material.mainTexture = Round1; 
 GUI.DrawTexture(Rect((Screen.width/2) - XRound1, YRound1, Round1Length, Round1width), Round1); 
 yield WaitForSeconds(5);
 LetsGetIt.rend.material.mainTexture = LetsGetIt; 
 GUI.DrawTexture(Rect((Screen.width/2) - x, y, LetsGetItLength, LetsGetItWidth), LetsGetIt);
 }