How do I fix error " 'UIPopoverController' is deprecated"?

Question

Updating an old iOS program. Getting this error message:

'UIPopoverController' is deprecated: first deprecated in iOS 9.0 - UIPopoverController is deprecated. Popovers are now implemented as UIViewController presentations. Use a modal presentation style of UIModalPresentationPopover and UIPopoverPresentationController.

Here is the code I'm working on:

#import <UIKit/UIKit.h>

@protocol SubstitutableDetailViewController
- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;
- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;
@end

@interface RootViewController : UITableViewController <UISplitViewControllerDelegate> {

    UISplitViewController *splitViewController;    
    UIPopoverController *popoverController;    
    UIBarButtonItem *rootPopoverButtonItem;
}

@property (nonatomic, assign) IBOutlet UISplitViewController *splitViewController;

@property (nonatomic, retain) UIPopoverController *popoverController;
@property (nonatomic, retain) UIBarButtonItem *rootPopoverButtonItem;

- (void) didSelectMenuItem:(NSNotification *) notification;
@end

Thanks in advance for any help, Greg