Change the selected TabBar index on button click

In my app I have 4 tabs, on the first Tab : I have 4 views. on second view,there is one button which will open the third tab view. But my selected tab Index is One. How can I chage it to Third.

My code for tab bar is in APPDelegate as follow:

  - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { 
     UINavigationController *nav1 = [[UINavigationController alloc] init];
         UIViewController *viewController1 = [[[CarAccidentAppViewController alloc]
     initWithNibName:@"CarAccidentAppViewController_iPhone" bundle:nil]
     autorelease];
        nav1.viewControllers = [NSArray arrayWithObjects:viewController1, nil];

         //for steps tab...

         UINavigationController *nav2 = [[UINavigationController alloc] init];
         UIViewController *viewController2 = [[[FirstSteps alloc] initWithNibName:@"FirstSteps" bundle:nil] autorelease];
         nav2.viewControllers = [NSArray arrayWithObjects:viewController2, nil];

         //for profiles tab...

        UINavigationController *nav3 = [[UINavigationController alloc] init];
         UIViewController *viewController3 = [[[Profiles alloc] initWithNibName:@"Profiles" bundle:nil] autorelease];
         nav3.viewControllers = [NSArray arrayWithObjects:viewController3, nil];

         //for contact us tab...

        UINavigationController *nav4 = [[UINavigationController alloc] init];
        UIViewController *viewController4 = [[[ContactUs alloc] initWithNibName:@"ContactUs" bundle:nil] autorelease];
         nav4.viewControllers = [NSArray arrayWithObjects:viewController4, nil]; self.tabBarController = [[[UITabBarController alloc] init]
     autorelease];
         self.tabBarController.viewControllers = [NSArray arrayWithObjects:nav1,nav2,nav3,nav4 ,nil];

         self.window.rootViewController=self.tabBarController;
        [self.window makeKeyAndVisible];
         return YES; 

}