You cannot force the app to update, but you can check if the current version is the latest and if not refuse to do anything until the app does update. This is typically necessary only if the data communication between the app and a server has changed. See
Linking to your app to explicitly go to your app's page in the store.
Windows 8.1 defaults to automatically updating apps without the user explicitly going to the store, but the user can turn this off.
--Rob