후킹이라기보다 교체? ㅋㄷ

방법이 많음..

UIBarButtonItem *backButton = [[UIBarButtonItem alloc]
    initWithTitle:@"Back"
    style:UIBarButtonItemStyleBordered
    target:self
    action:@selector(backButtonTapped:)];

self.navigationItem.leftBarButtonItem = backButton;

UIBarButtonItem *backButton = [[UIBarButtonItem alloc] 
    initWithBarButtonSystemItem:UIBarButtonSystemItemRefresh
    target:self
    action:@selector(backButtonTapped:)];
self.navigationItem.leftBarButtonItem = backButton;

UIImage *backButtonImage = [UIImage imageNamed:@"back_arrow.png"];
UIButton *leftBarButton = [UIButton buttonWithType:UIButtonTypeCustom];

// Setting the image for the back button
[leftBarButton setBackgroundImage:backButtonImage forState:UIControlStateNormal];
[leftBarButton addTarget:self action:@selector(backButtonTapped:) forControlEvents:UIControlEventTouchUpInside];

// setting the frame for the back button
leftBarButton.frame = CGRectMake(0, 0, 65, 32);

// setting the custom button to navigationbars back button
self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:leftBarButton];