Official Android 6.0 Marshmallow update has been released for AT&T version of LG G4 in early February 2016, following the other US variants like T-Mobile and Sprint, which are the first to get it. The update was released with software version H81020n, available via FOTA (Firmware Over the Air) and using Wi-Fi connection. Unfortunately, there’s no way to root Marshmallow updates on the AT&T LG G4 (H810) at the moment.
The H81020n update is the fourth update received by this smartphone, the previous software version was 10o, released in October 2015 that brought several enhancements. According AT&T support page, in addition to Android OS upgrade, the 20n software update also brings improvements to video calling feature. Since it is a major OS upgrade, the update file size is quite large, at around 970MB, that’s why you are required to download it over Wi-Fi.
Regarding root on the AT&T LG G4 running official Android 6.0, we still have not found any working method for the latest update. As we know, if the AT&T version (H81020n) has been sold with locked bootloader that makes any root attempts more challenging, and up to now no one has been able to unlock it.
Previously, we could use a method by Team Codefire called “Low Effort Root”, it’s require a rootable system image. Unfortunately, the rootable H81020n system image is not available yet, so we can not use those methods to achieve superuser privileges.
So, for now you have to choose one of the two options, keep the phone running rooted Lollipop, or update it to the official Android 6.0 without root and waiting someone to share its pre-rooted Marshmallow system image.