How to switch between windows ( if not from Start ).

[Frederick Douglass]

In my program I have two separate windows and I want to be able to switch between them easily via a button on each window.
I even managed to get to the second screen via a button on the first. The question is how do I get back to the first screen, given that it was defined as :

 public void start(Stage primaryStage) {
// create a window with a vbox layout, set its size and title ( Main Box = mb )
    primaryStage.setTitle("Interest Calculator");   
    VBox  mb = new VBox();
    mb.setFillWidth(true);
    primaryStage.setScene(new Scene(mb, 600, 350));
    primaryStage.show();

etc. ?

As I see it, the problem is that this is in the start routine and so it is not a method than can be executed via a line of code elsewhere in the program.
Rightly or wrongly I am used to setting up windows in this way ( via the start routine ) but this is my first time with a second window.
I'm afraid that at worst I would have to rewrite from scratch with the code shown above in its own method,
executed via a line of code elsewhere in the program. At best it will be a simple case of saying

    
primaryStage.show();

but even if so, where exactly can I place that ? Thank you.