Swift- How can I have a button refresh each time it's hit?

I can't seem to figure out how to have a function block repeated when a UIButton is pressed in swift. Here's what I have:

class CoinFlip: UIViewController {
@IBOutlet var resultLabel: UILabel!
    var randomNumber = (Int(arc4random_uniform(2)))
    @IBAction func tossButton(sender: UIButton) {
        if randomNumber == 0 {
        resultLabel.text = "Heads!"
        }
        else if randomNumber == 1 {
        resultLabel.text = "Tails!"
        }
    }
}

When the button is pressed in the app, it picks a random number. If I hit it again, it displays the same number. What can I do so each time I hit the button it refreshes?