Синтаксис SWIFT 3
let str = NSMutableAttributedString(string: "First line\nSecond Line")
str.addAttribute(NSFontAttributeName, value: UIFont.systemFont(ofSize: 17), range: NSMakeRange(0, 10))
str.addAttribute(NSFontAttributeName, value: UIFont.systemFont(ofSize: 12), range: NSMakeRange(11, 11))
button.setAttributedTitle(str, for: .normal)