Code:
// class names (linkage) of screens in library
var _screens:Array = ["Screen1","Screen2","Screen3","Screen4","Screen5"];
// instance names of btns in the btnMenu
var _btns:Array = ["btn1","btn2","btn3","btn4","btn5"];
// class name (linkage) of menu in library
var _menu:MovieClip = new BtnMenu();
var _base:MovieClip = new MovieClip();
var _current:MovieClip;
var _num:int = 0;

updateScreen();

function updateScreen($e:Event=null):void {
        var classRef:Class;
        var tween:Tween;
        var newScreen:MovieClip;
        if ($e == null) {
                classRef = getDefinitionByName(_screens[0]) as Class;
                _current = new classRef();
                _base.addChild(_current);
                newScreen = _current;
                addChild(_base);
                addChild(_menu);
                _menu.addEventListener(MouseEvent.CLICK, updateScreen);
        } else {
                var num:int = _btns.indexOf($e.target.name);
                if (num != -1 && num != _num) {
                        classRef = getDefinitionByName(_screens[num]) as Class;
                        newScreen = new classRef();
                        _base.addChild(newScreen);
                        if (num > _num) {
                                newScreen.y = _current.height;
                                new Tween(_current,"y",Strong.easeOut,0,0 - _current.height,1,true);
                                tween = new Tween(newScreen,"y",Strong.easeOut,newScreen.y,0,1,true);
                        } else {
                                newScreen.y = 0 - newScreen.height;
                                new Tween(_current,"y",Strong.easeOut,0,_current.height,1,true);
                                tween = new Tween(newScreen,"y",Strong.easeOut,newScreen.y,0,1,true);
                        }
                        tween.addEventListener(TweenEvent.MOTION_FINISH, function(){_base.removeChildAt(0);});
                        _num = num;
                        _current = newScreen;
                }
        }
}