AT&T suggests that you back up your data prior to downloading the update, even though it should have no effect on your personal content. The carrier also expects you to use a Wi-Fi network when updating your LG G Flex. And you probably should also have your battery recharged to at least 50%.
The update won't hit every AT&T customer with the LG G Flex on Thursday. These updates are sent out in waves, so if you don't get it tomorrow, you will get it soon. You can also try to coax the update out of your phone manually by going to Settings > General tab > About phone > Software update > Update now.
LG G Flex
source: AT&T via DroidLife