How do i unroot my device?

[AC Question]

Hello. I wish to update my Samsung galaxy tab 3 8.0 (sm-t311) but whenever i try to update it says my device has been modified and that there will will be no updates for me. Even my device status from Settings>About shows device status as "custom". But I have never rooted my device nor have I done anything to modify my device. So, to make sure, I installed Root Checker to see if my device has actually been rooted. When I ran the app, it show that "root access has not been oroperly installed on this device". Now I have no idea as to whether my tab is rooted or not. I want to return my tab to its stock, original, normal, default state. How can I get this done? Thanks!