UIBarButtonItem *backItem = [[UIBarButtonItem alloc] initWithTitle:@"返回上级界面" style:UIBarButtonItemStylePlain target:nil action:nil];    

self.navigationItem.backBarButtonItem = backItem;

UIBarButtonItem *backItem = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"箭头left40x40"] style:UIBarButtonItemStylePlain target:nil action:nil]; 

self.navigationItem.backBarButtonItem = backItem;