For iOS devices:
If you have selected automatic updates, the app will update on its own when updates are available. If you do not have automatic updates turned on, or if the app didn’t update, you can manually update the app.
If a manual update is required, see How to manually update apps on your Apple device for more information.
For Android devices:
If you have selected auto update, the app will update on its own when updates are available. If auto updates are not enabled, or if your app is not updating, you can manually update the app.
If a manual update is required, see How to update apps on Android for more information.