Android 8.0:
In Android 8.0 "Unknown Sources" option has been replaced by another option. Now
when you would like to install an app (APK) with any app other than the Google
Play Store, it will show you a popup and which will take to you “External
Sources”. From there you can allow third-party apps installation permission for
that app. So when you want to install an app in the future then installation
will not be blocked for that app.
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Android 7.0:
In Settings tap on Security (Lock screen and security).
In Security tap on Unknown sources toggle button.
A small verification screen will appear tap on OK option.
Now Unknown sources option has been activated in Phone.
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Android 6.0:
Settings >> Lock screen and security >> tap on toggle button of Unknown Sources.
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Android 5.1:
Go to Settings >> Security >> Unknown sources.
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Android 4.4.2:
Settings >> Tap on Security Option >> Check Unknown sources checkbox…
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Android 4.2:
Start Setting >> Security >> Unknown Sources.