i think once root, cannot use OTA already, must update manually.

you already can do root, should not be novice already.  But what OTA update you need really?  Or just want to update for the sake of update?

Anyway, you can go sammobile.com and register a free account.  Thereafter can download the official ICS ROM from there and flashed with PC Odin.  You will lose root after flashing, just do the rooting again will do.  But i am not sure whether flashing via PC Odin will wipe clean your phone, so you better backup.