How do I set title for UIBarButtonItem?

Use different initialiser that allows you to specify the title:

UIBarButtonItem(title: "title", style: .Plain, target: self, action: "barButtonItemClicked:")

Swift 3.1 Update

UIBarButtonItem(title: "title", style: .plain, target: self, action: #selector(barButtonItemClicked))

Swift 2 answer

You could just add the following to viewDidLoad:

// Some text
self.barButtonItemClicked.title = "Right Button!"

OR

// A Unicode Gear Symbol
// See: http://graphemica.com/%E2%9A%99
self.barButtonItemClicked.title = "\u{2699}"

The ViewController.Swift code below would set the name barButtonItemClicked you used. I used a UIBarButtonItem to show how to set the title. But it is trivial to adapt it for a function.

import UIKit

class ViewController: UIViewController {

    // Please make sure the line below is properly connected to your Storyboard!
    @IBOutlet weak var barButtonItemClicked: UIBarButtonItem!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

       // Set the text of the NavBarButtonItem Title  
       self.barButtonItemClicked.title = "Right Button!"

       // Gear Icon
       // self.barButtonItemClicked.title = "\u{2699}"

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

}