Fortunately it's easy. The problem is where to do it.
Every UIViewController has a default back button, but it never uses its own. Back button you see on-screen belongs to the previous view. In other words, viewController's back button is used only when returning to this view from elsewhere. Therefore when you want to change back button, do it in the "previous" view.
- (void)viewDidLoadOther ways:
UIBarButtonItem *backButton =
self.navigationItem.backBarButtonItem = backButton;
- You can change previous view title in viewDidDisappear, but unfortunately you can see back button text change on-screen. Additionally you need to restore original title in viewWillAppear.
- You can define custom leftBarButtonItem in current view's viewWillAppear, but button shape will be rectangle. Normally back button has arrow like shape pointing left. You could fix this by using custom graphics as button image(s).