Android® 8.x & higher

1. From a Home screen, swipe up or down from the center of the display to access the apps screen.
2. Navigate: Settings  > Apps.
3. Tap Menu icon (upper-right).
4. Tap Special access.
5. Tap Install unknown apps.
6. Select the unknown app then tap the Allow from this source switch to turn on  or off .

Android® 7.x & lower

1. From a Home screen, navigate to Settings.
2. Tap Lock screen and security.
    If unavailable, tap Security.
3. Tap the Unknown sources switch to turn on or off.
    If unavailable, Unknown sources to turn on or off. Enabled when check mark is present.
4. To continue, review prompt then tap OK.