This examples shows how to swith between  full screen

function switch_FullScreen(evt:event):void {
 if (stage.displayState== "normal") {
 stage.displayState="fullScreen";
 stage.scaleMode = StageScaleMode.NO_SCALE;
 } else {
 stage.displayState="normal";
 }
}

button.addEventListener(MouseEvent.CLICK, switch_FullScreen);