From your smartphone or tablet running Android 4.0 or higher
Selecting this option will allow you to install apps outside of the Google Play store.
Depending on your device, you can also choose to be warned before installing harmful apps.
This can be enabled by selecting the Verify apps option in the Security settings.
On devices running an earlier version of Android, go to Settings, open the Applications option, select Unknown sources, and click OK on the popup alert.