IPhone : Change a Nav Bar button icon in Swift

Im trying to change the button icon PLAY for PAUSE when I tap on it. But my code is not working. Please help!

This is my @IBAction code.

@IBAction func playTimer(sender: AnyObject) {

    if buttonSwitch == 0 {
        timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: Selector("result"), userInfo: nil, repeats: true)
        buttonSwitch = 1

        self.navigationItem.setLeftBarButtonItem(UIBarButtonItem(barButtonSystemItem: .Pause, target: self, action: "setAlarm"), animated: true)

        println("2")

    } else {
        timer.invalidate()
        buttonSwitch = 0
    }

}

I tried everything but nothing's happened. No errors, the icon does not change.

Thanks.