In main.dart (or any) file, first create an instance of the UpdateNotification class in your initState() method.
final UpdateNotification updateNotification = UpdateNotification();
Pass the application package name in andoidAppId, iOSAppId.
Pass application minimum version value in minimumVersion parameter. This parameter is used to force people to update application, any user having version less than the specified version will be forced to update application
If updated app version is available on stores, application will get a popup on launch of application and an option to Update. On click of update button user will be directed to the respective stores(App store or play store).