ENABLE INSTALLING "UNKNOWN APPS" ON ANDROID NOUGAT
Go into Settings.


Tap Security (or Lock Screen and Security).

Scroll down to the Device Administration section, and enable Unknown Sources.

ENABLE INSTALLING "UNKNOWN APPS" ON ANDROID OREO
Go to Settings.

Than Apps and Notifications.

Select Install Unknown Apps (or Install Other Apps).

Install an Android File Manager
Now that you've configured your phone to let you install unknown apps, you'll also need a way to find that application file (APK file) on your phone so you can run it.

Android phones typically come with a "File Manager" app you can use. But if you don't have one, then open Google Play on your phone and search for "File Manager." You'll see lots of options available.

Some of the best options include:

Cx File Explorer
EZ File Explorer
File Manager

Install any of these file managers so you'll be able to locate the APK file after you've transferred it to your phone.