Modify Navigation Controller stack

// embed FirstVC as the Root View Controller of a Navigation Controller

class FirstVC: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .yellow

        let button = UIButton(type: .system, primaryAction: UIAction(title: "Push to Second VC", handler: { _ in
            let vc = SecondVC()
            self.navigationController?.pushViewController(vc, animated: true)
        }))
        view.addSubview(button)
        button.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])

    }

}

class SecondVC: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .green

        let button = UIButton(type: .system, primaryAction: UIAction(title: "Push to Third VC", handler: { _ in
            let vc = ThirdVC()
            self.navigationController?.pushViewController(vc, animated: true)
        }))
        view.addSubview(button)
        button.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])

    }

}

class ThirdVC: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .cyan

        let v = UILabel()
        v.text = "This is Third VC"
        view.addSubview(v)
        v.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            v.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            v.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])


        if let navC = self.navigationController {
            var a = navC.viewControllers
            a.remove(at: 1)
            navC.setViewControllers(a, animated: false)
        }
    }

}