Given I747 has 4.1.2 but downloading something

Hey all, have the above mentioned phone, been wiped clean and factory reset, SD card blank. But when connected to internet it starts downloading an update from Att. Now I want to root, from what I've read bootloader is unlocked, and then twrp and ROM etc. Should I take the update then root or better to root and do incremental updates then ROM. Tremendous amount of info here and am still a little unsure which way to go. Thanks

Sent from my DROID Turbo using XDA-Developers mobile app