Switching the Root of NavigationController（Swift4.2）

	import UIKit

	@UIApplicationMain
	class AppDelegate: UIResponder, UIApplicationDelegate {

	var window: UIWindow?
	var navigationController: UINavigationController?

	func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
	self.window = UIWindow(frame: UIScreen.main.bounds)
	self.window!.makeKeyAndVisible()
	let firstViewController: FirstSwitchNavigationViewController? = FirstSwitchNavigationViewController()
	navigationController = UINavigationController(rootViewController: firstViewController!)
	window!.rootViewController = navigationController
	return true
	}

	func applicationWillResignActive(_ application: UIApplication) {
	}

	func applicationDidEnterBackground(_ application: UIApplication) {
	}

	func applicationWillEnterForeground(_ application: UIApplication) {
	}

	func applicationDidBecomeActive(_ application: UIApplication) {
	}

	func applicationWillTerminate(_ application: UIApplication) {
	}
	}

view raw AppDelegate.swift hosted with ❤ by GitHub

	import UIKit

	class FirstSwitchNavigationViewController: UIViewController {

	override func viewDidLoad() {
	super.viewDidLoad()

	title = "FirstView"
	view.backgroundColor = UIColor.red

	let width = view.frame.width
	let height = view.frame.height

	let transitionBtn = UIButton()
	transitionBtn.frame.size = CGSize(width: width * 0.6, height: height * 0.2)
	transitionBtn.center = view.center
	transitionBtn.backgroundColor = UIColor.blue
	transitionBtn.setTitle("Switch to SecondVC", for: UIControl.State.normal)
	transitionBtn.addTarget(self, action: #selector(goSevondVC(sender:)), for:.touchUpInside)
	view.addSubview(transitionBtn)
	}

	@objc internal func goSevondVC(sender: UIButton){
	let secondVC = SecondSwitchNavigationViewController()
	navigationController?.setViewControllers([secondVC], animated: true)
	}
	}

	import UIKit

	class SecondSwitchNavigationViewController: UIViewController {

	override func viewDidLoad() {
	super.viewDidLoad()

	title = "SecondView"
	view.backgroundColor = UIColor.blue

	let width = view.frame.width
	let height = view.frame.height

	let transitionBtn = UIButton()
	transitionBtn.frame.size = CGSize(width: width * 0.6, height: height * 0.2)
	transitionBtn.center = view.center
	transitionBtn.backgroundColor = UIColor.green
	transitionBtn.setTitle("Switch to FirstVC", for: UIControl.State.normal)
	transitionBtn.setTitleColor(UIColor.black, for: UIControl.State.normal)
	transitionBtn.addTarget(self, action: #selector(backToFirstVC(sender:)), for:.touchUpInside)
	view.addSubview(transitionBtn)
	}

	@objc internal func backToFirstVC(sender: UIButton){
	let firstVC = FirstSwitchNavigationViewController()
	navigationController?.setViewControllers([firstVC], animated: true)
	}
	}