ios – Programmatically changing the properties of a VC’s navigation bar button in response to a button press in another VC

I’ve created a navigation bar button in MainViewController using

override func viewDidLoad() {
    super.viewDidLoad()    

    navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Save", style: .plain, target: self, action: #selector(presentPopup))
        
}
    
@objc func presentPopup() {
        
    let popupVC = PopupViewController()
    popupVC.modalPresentationStyle = .overCurrentContext
    present(popupVC, animated: true, completion: nil)
        
}

which presents a popup form (containing a “save” button) used for saving the main view’s contents. I’d like for this “save” button (in PopupViewController) to (1) change the title of MainViewController‘s navigation bar button to “Saved” and (2) disable the button.

I have the following in PopupViewController so far:

let popupBox: UIView = {
    let view = UIView()
    view.translatesAutoresizingMaskIntoConstraints = false
    return view
}()

let button: UIButton = {
    let view = UIButton()
    view.translatesAutoresizingMaskIntoConstraints = false
    return view
}()

override func viewDidLoad() {
    super.viewDidLoad()
        
    self.definesPresentationContext = true
    view.backgroundColor = .clear
        
    view.addSubview(popupBox)
    // configure popupBox

    view.addSubview(button)
    // configure button
    button.addTarget(self, action: #selector(buttonAction), for: .touchUpInside)
        
}

@objc func buttonAction(_ sender: UIButton) {
    // save main VC's contents
    // change title of main VC's nav button - WHAT GOES HERE?
    // disable main VC's nav button - WHAT GOES HERE?
    view.removeFromSuperview()  // dismiss popup
    
}

What should I add to the button’s action to accomplish what I’m trying to do?