Pausing issue with OnMouseDown

I have several gui textures in my game and the following pause/restart script is on one of the buttons just to toggle on and off. However the script works when the mouse is clicked anywhere. I know it means I have to replace Input.GetMouseButtonUp(0) with function OnMouseDown but how do I do it? Getting too many errors. I know it’s a simple thing but I can’t find the answer.

var paused : boolean = false;

function Update () { if(Input.GetMouseButtonUp(0)){ if(paused == true){ paused = false; } else { paused = true; }

     if(paused == true){
         Time.timeScale = 0.0;
     } else {
         Time.timeScale = 1.0;
     }
 }

}