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(); startText = startText.GetComponent(); 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(); } }