There are possibly multiple ways in which a user may do this. First, they will have to visit the original homepage of their device and then run their finger on the screen by swiping upwards or downwards on it. That action should show the place that has all the applications. Once the user is there, they may find the settings option on the search bar or find it manually. Then, once the user presses on it, they may select the menu option and follow it up by visiting the place that mentions special access. There, the user may search for the option that sanctions the installation of these applications from places that are not common on their device, and they must now press it to allow entry. Once that is finished, they may choose the application they want to keep from an unknown place. Then, they may have to provide entry to the switch for the origin, which can now be opened or closed at any given time. This should fix the App Not Installed error.