UIBarButtonItem *temporaryBarButtonItem = [[UIBarButtonItem alloc] init];
temporaryBarButtonItem.title = tmpString; // this is your text to be set
self.navigationItem.backBarButtonItem = temporaryBarButtonItem;
UIBarButtonItem *temporaryBarButtonItem = [[UIBarButtonItem alloc] init];
temporaryBarButtonItem.title = tmpString; // this is your text to be set
self.navigationItem.backBarButtonItem = temporaryBarButtonItem;