//Very simple solution is here

//To change page (UIViewController) from a UIViewController, first get the instance of Parent ViewController (which will be UIPageViewController) and then set its current ViewController

//In my case I have a UIPageViewController named "UserDetailsPVC" and it contains 4 pages (UIViewControllers) which are as follows

  PageOneVC:UIViewController
  PageTwoVC:UIViewController
  PageThreeVC:UIViewController
  PageFourVC:UIViewController

//In the UIPageViewController lets define an array of pages

 var pages:[UIViewController] = [
        UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "PageOneVC"),
        UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "PageTwoVC"),
        UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "PageThreeVC"),
        UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "PageFourVC")
]

//Now to change page of UIPageViewController from any of the UIViewController

    // get parent view controller
    let parentVC = self.parent as! UserDetailsPVC

    // change page of PageViewController
    parentVC.setViewControllers([parentVC.pages[1]], direction: .forward, animated: true, completion: nil)