How to quit game on double pressing back button?

I am trying to quit my game after pressing back button but its not working. Here is my code:

 public Canvas quitMenu;
 public Button startText;
  
 // Use this for initialization
 void Start ()
 {
     quitMenu = quitMenu.GetComponent<Canvas>();
     startText = startText.GetComponent<Button>();

     quitMenu.enabled = false;
 }
 void Update()
 {
     if (Input.GetKeyUp(KeyCode.Escape))
     {
         quitMenu.enabled = true;
     }
 }
 public void exitPress()
 {
     quitMenu.enabled = true;
     startText.enabled = false;
 }

 public void noPress()
 {
     quitMenu.enabled = false;
     startText.enabled = true;
 }

 public void startLevel()
 {
     SceneManager.LoadScene("Scene1");
 }

 public void exitGame()
 {
     Application.Quit();
 }

}