JavaFX window setTitle

Exposing properties using static, just for the sake of exposing, may be considered as a bad design. You have different ways to achieve the same, for example, expose a method from the Window class which sets the stage title.

public class Window extends Application {

    private Stage stage;

    @Override
    public void start(Stage foablak) throws Exception {
        stage = foablak;
        Parent root = FXMLLoader.load(getClass().getResource("Foablak.fxml"));
        Scene scene = new Scene(root);

        foablak.setScene(scene);
        foablak.setWidth(900);
        foablak.setHeight(700);
        foablak.setResizable(false);
        foablak.setTitle("Window");

        foablak.show();
    }

    public static void main(String[] args) {
        launch(args);
    }

    public void setStageTitle(String newTitle) {
        stage.setTitle(newTitle);
    }
}

Yes, you can. Inside of your Application.start() method, save a reference to your primary Stage that you can access elsewhere, and then call Stage.setTitle().

class MyApplication extends Application {

    public static Stage primaryStage;

    @Override
    public void start(Stage primaryStage) {
        MyApplication.primaryStage = primaryStage;

        // ...
    }

}

MyApplication.primaryStage.setTitle("New Title");

As an aside, I would avoid calling your class Window, as that is the name of one of the JavaFX classes.