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