Don't Want G6 To Update

Just received off eBay a G6 H872 with ver. 7.0. I am running it on a AT&T mvno. The thing it has all the Tmobile stuff, before I could disable automatic checks it downloaded an update which I believe is ver. 8.0. Alert says touch to restart and complete update. I first want to get down the process to root before upgrading, and because of anti rolll back, yada yada I obviously don't want it to update.
Is it possible to delete the downloaded update?
If the phone restarts for any reason at all is the update gonna get installed, or doe the update only run if you select to restart from the update notification?