Uninstalling updates restores the version of an app that is "baked in" to your device's operating system. This is possible because apps included out of the box on a device are stored on the protected system partition, so any updates you apply to them through the Play Store (or other source) are installed alongside these originals rather than overwriting them. As a result, removing the newer version causes your device to revert to the system app.
When you install an OS update, however, the system partition app versions are generally updated to whatever their latest iterations were when the OS version was put together. You can still uninstall future updates, but you can't go back to a version earlier than that baked in to the OS.
Old APKs are available online, but to my understanding you will have to root your phone and remove the more-recent version now on your system partition before you can use an older version of the app. I haven't ever done this, though, so that may be incorrect.
07-26-2013 06:45 PM