Как изменить свойство primaryStage.setTitle

Необходимо при нажатии кнопки на форме изменять значение primaryStage.setTitle();

Main.java

package sample;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
public class Main extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("view.fxml"));
        primaryStage.setTitle("Исходная форма");
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
        primaryStage.setResizable(false);
    }
    public static void main(String[] args) {
        launch(args);
    }
}

Controller.java

package sample;
import javafx.application.Application;
import javafx.fxml.FXML;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Tooltip;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;

public class Controller {
    @FXML
    public Button btnRed = new Button();
    @FXML
    public Button btnYellow = new Button();
    @FXML
    public Button btnGreen = new Button();
    @FXML
    public Pane PaneM = new Pane();
    Tooltip tooltipRed = new Tooltip();
    Tooltip tooltipYellow = new Tooltip();
    Tooltip tooltipGreen = new Tooltip();
    public void tooltipsValues() {
        tooltipRed.setText("RED");
        tooltipGreen.setText("GREEN");
        tooltipYellow.setText("YELLOW");
    }
    public void initialize(){
        btnRed.setTooltip(tooltipRed);
        btnYellow.setTooltip(tooltipYellow);
        btnGreen.setTooltip(tooltipGreen);
    }
    public void onClickRed() {
        PaneM.setStyle("-fx-background-color: #7e1300");
        btnRed.setDisable(true);
        btnGreen.setDisable(true);
        btnYellow.setDisable(false);
    }
    public void onClickYellow(){
        PaneM.setStyle("-fx-background-color: #7e7d10");
        btnYellow.setDisable(true);
        btnRed.setDisable(true);
        btnGreen.setDisable(false);
    }
    public void onClickGreen(){
        PaneM.setStyle("-fx-background-color: #2d8e26");
        btnGreen.setDisable(true);
        btnYellow.setDisable(true);
        btnRed.setDisable(false);
    }
}