Unhappy [Q] Uninstalling custom rom to go back to factory settings

So the other day i bought the note 2 form someone. I did not realize it had a custom rom on it until i got the 4.3 update and could not proceed with the download because for interruptions. When I found out it wasnt the orginal one from ATT I was thinking I know there was something that could be done.

Most likely the person was trying to unlock the phone and downgraded with a custom rom and tried to update OTA.

How could i uninstall the custom rom and go back with the factory firmware.
My phone is in odin mode with custom and knox cout is 1.
Would I have to root my phone to do it or could i go into the setting using the down vol, home and power btn and wipe clean or would that mess everything up.
Also my phone have bad signal problems when in certain houses lol .

HELPPPPPPP

this is what my phone say.
Attached Thumbnails
Click image for larger version

Name:	Screenshot_2014-06-25-19-47-23.png
Views:	55
Size:	144.4 KB
ID:	2817754