hide the default navigation "back" button
self.navigationItem.hidesBackButton = YES;
show the default navigation "back" button
reference:
http://stackoverflow.com/questions/5085945/disable-backbutton-when-navigation-controllers-pushed-via-a-tab-bar-controller
self.navigationItem.hidesBackButton = NO;
reference:
http://stackoverflow.com/questions/5085945/disable-backbutton-when-navigation-controllers-pushed-via-a-tab-bar-controller