How do I reset Galaxy Mega to stock & unrooted?

[GranPaSmurf]

I need to take my Galaxy Mega to the phone shop. Months back I rooted the phone. Now I'm trying to get it back to unrooted stock. I did the factory reset back to original Samsung. It looks original but when I go to the update page and try to get any updates that might have come along since I rooted the phone, it still tells me the phone has been modified in an unauthorized way.
If I take it to the shop, they will just turn me away.
Do you know of a way I can get it REALLY back to factory installation?

 

[SylviaGb]

Hi, I wolud Llike to know How and with what did you root your phone, Grretings and Thanks