The best way to do this is to create a new LoginViewController as other people have mentioned and then set your rootviewController to tabBarcontroller as soon as you authenticate the user. Here is how you can do this in swift, this is snippet to put as soon you authenticate your user in LoginViewController
let tabBarController = self.storyboard?.instantiateViewControllerWithIdentifier("TabBarController") as! UITabBarController
let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
appDelegate.window?.rootViewController = tabBarController
Where TabBarController is the storyboard id of your tab bar controller. It could be anything whatever name you have given it.