Hidden in AppDelegate.cpp, you've had lines like:

auto scene = GameLayer::scene();
// run
director->runWithScene(scene);

So, in order to change scenes, all you need to do is tell the Director class which scene you wish it to run. Cocos2d-x will then get rid of all the content in the current scene, if any (all their destructors will be called), and a new layer will be instantiated and wrapped inside the new Scene.

Breaking the steps down a little further, this is how you usually create a new scene for Director:

Scene* MenuLayer::scene()
{
    // 'scene' is an autorelease object
    auto scene = Scene::create();
    // add layer as a child to scene
    auto layer = new MenuLayer();
    scene->addChild(layer);
    layer->release();
    return scene;
}

The static MenuLayer::scene method will create a blank scene, and then create a new instance of MenuLayer and add it as a child to the new scene...