set title of back bar button item in ios

I am using storyboards in my application, and if I set the title property of a ViewController class, that string will appear as the text of my back button when I push a SecondViewController, how can I change this?, I want to put a different title for the back button.

I have used this but it doesn't work:

UIBarButtonItem *btnBack = [[UIBarButtonItem alloc]
                                   initWithTitle:@"Back" 
                                   style:UIBarButtonItemStyleBordered
                                   target:self
                                   action:nil];
    [self.navigationItem setBackBarButtonItem: btnBack];

please help me

Try this ,

Objective-C:

UIBarButtonItem *btnBack = [[UIBarButtonItem alloc]
                            initWithTitle:@"Done"
                            style:UIBarButtonItemStyleBordered
                            target:self
                            action:nil];
self.navigationController.navigationBar.topItem.backBarButtonItem=btnBack;

Swift:

var btn = UIBarButtonItem(title: "Done", style: .Plain, target: self, action: "backBtnClicked")

self.navigationController?.navigationBar.topItem?.backBarButtonItem=btn