Apple (iOS)
Apple (via the App Store) will notify your device when there is an update available
Android
Android (Via the Google Play Store) by default will automatically update apps using Wi-Fi connection.
If you turn off 'auto-updates' in the Play store, you will receive notifications for pending updates.
You can always check for updates by opening the Google play store - tapping your profile & selecting 'manage apps & device'
Note: if you have previously downloaded the app using the desktop QR code, you must remove the application from your device & re download it from the Play Store.