You can revert the version of an installed application if an earlier version exists on the server.
See Preparations for using the mobile client for information about how to disable reverting to earlier versions of an application.
If the installed version of the Android operating system is earlier than 4.2.2, tap Revert.
If the installed version of the Android operating system is 4.2.2 or later, you must uninstall the current version before you can install the earlier version.
Then, follow the procedure documented in Installing an application on an Android device.
Use the normal procedure of the operating system to remove the application.
Tap Install to install the earlier version of the application. Follow the procedure documented in Installing an application on an iOS device.