Change UINavigationBar back button title

self.navigationController.navigationBar.topItem.title = @"";

Do this in the parent view controller not in the child

Swift

navigationItem.backBarButtonItem = UIBarButtonItem(title: "Back", style: .plain, target: nil, action: nil)

Objetive-C

self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"Back" style:UIBarButtonItemStylePlain target:nil action:nil];