For iOS devices, beta versions are distributed via Apple's TestFlight app. After you sign up, you will receive an invitation e-mail from Apple's TestFlight. Install the TestFlight app and then accept the invitation, and you will be able to install the beta version over your existing App Store install. You can install the App Store version back over top of the beta version at any time.
For Android devices, beta versions are distributed via Google Play. Simply click the Google Play Open Beta Testing link above to Become a Tester.
For iOS devices, beta versions expire after 90 days. Each beta update must be manually installed. If a beta version expires, the app will no longer launch until a new beta version or the App Store release version is installed over it. Beta apps will not be automatically updated to any more recent versions released via the App Store. This means that you might need to manually install the App Store version if your beta is about to expire and there is not a more recent beta available yet.
For Android devices, beta versions do not expire. Installed beta versions will be automatically updated to any more recent beta or release versions released via Google Play.