How can I do OS updates on my Samsung galaxy s3 (have rooted and unrooted phone)

[AC Question]

I need help as I rooted the my Samsung galaxy s3 and I tried to update the OS then it said it was modified and it failed. I then decided to unroot it and it gave me the exact same message again what must I do

 

[anon(632115)]

How did you "unroot" it?