How to change the state of an UIButton in iOS using Swift?

I have Objective-C working code for six buttons but I don't seem to find how to change the state of a button using Swift. So how do I make the following code work in Swift?

[self.button2 setBackgroundImage:[UIImage imageNamed:@"button2_selected.png"] forState: UIControlStateSelected];
button1.selected = NO;
button2.selected = !button2.selected;
button3.selected = NO;
button4.selected = NO;
button5.selected = NO;
button6.selected = NO;