nametakeAppDelegate.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  // vc1をrootにしてUINavigationControllerの初期化
  UIViewController *vc1 = [[UIViewController alloc] init];
  UINavigationController *nc = [[UINavigationController alloc] initWithRootViewController:vc1];

 // vc2で切り替える
  UIViewController *vc2 = [[UIViewController alloc] init];
  [nc setViewControllers:@[vc2]];

  return YES;
}

thx !!!!!!!!