change the text of attributes string in uilable in swift : I am using attributes string to add some text to my uilable, and I want to change this text when user press the uilabel

here is my code 

            let likes: UILabel = UILabel(frame: CGRectMake(5, CGFloat(totalHeight + 5), 50, 21));
            let likesString: NSAttributedString = NSAttributedString (string: String(selectedItem.noLikes))
            var myString: NSMutableAttributedString = NSMutableAttributedString(string: " ")
            myString.appendAttributedString(likesString)
            likes.attributedText = myString
            mainScrollView.addSubview(likes)

what I want to change the value of `likesString` from `(selectedItem.noLikes)` to `(selectedItem.noLikes + 1)`

can any one help me please ?