My app registers for the notifications like this :
[[UIApplication sharedApplication] registerForRemoteNotificationTypes:UIRemoteNotificationTypeBadge | UIR
You can't. In iOS 5.0 there was a not documented feature to open the notification setting page (or any setting page) from your application via an url scheme. Then you could ask the user to enable the notifications. But it was removed in 5.1 and anyway probably would not be accepted by Apple.
registerForRemoteNotificationTypes
asks the user whether he wants to accept notifications from your application only the first time you call it. If later the user changes the notification settings then it will not ask it again any more. You can check whether notifications are enabled for your application with enabledRemoteNotificationTypes
and warn the user yourself if you want, but user will have to go manually to the settings and re-enable them. You can't do it any more from your application.
UPDATE as of iOS 8.0
From iOS 8.0 Apple returned the possibility to open the System Settings page of your app:
NSURL* settingsURL = [NSURL URLWithString:UIApplicationOpenSettingsURLString];
[[UIApplication sharedApplication] openURL:settingsURL];