operating system has been modified in an unauthorized way

[Woodstock13]

I have Samsung galaxy core 4.4.4 Android phone and when I hit 'software updates' I get a message telling me the operating system on my device has been modified in an unauthorized way. What can I do to get my system back to normal and be able to update? The device name of my phone says SM-S820L.

 

[Rukbat]

Have the stock ROM flashed to the phone. (Careful - you'll lose all your installed apps and all your data, so back them all up first - see Backing up an Android Device.)