The DJI Fly App is no longer available in the Play Store. In the
Smart Controller you can update the app by downloading it from DJI's downloads page on their web site using the web browser in the controller. The same applies to any smart phone. You can then use the file manager to find the .APK file and install it from there. Once you've updated to the most recent version, it should automatically start notifying you of updates in the future as they are published and it will download them in the background and then offer to install them.
Basically, DJI has a shorter release cycle than the Play Store allows them.