How do you change an IBAction UIButton's image in Swift?

    if(SelectedRestaurant == "Yo!Sushi, UK"){
        BowlOnePrice = YoUkGreenPrice
        Currency = Pounds

    }

@IBAction func BowlOne(sender: UIButton) {}

So I've got an if statement and a UIbutton linked as an IBAction. In the storyboard the button is an image. I want to be able to change the image in the if statement. But I can't access the "BowlOne" button. Does anyone know how?