It was only a little over a week ago that we heard that LG was planning a surprisingly quick update to Android 6.0 for its flagship G4 phone, beginning with Poland and expanding out to other territories. We haven't heard anything about an official rollout just yet, but someone's managed to get their hands on a Marshmallow build for the international unlocked version of the G4, model number H815. A KDZ file appeared on the forum yesterday, and it's been adapted into flashable files for users with the TWRP custom recovery already.
We haven't heard of any official over-the-air 6.0 updates for the G4, but the 20A build posted by "autoprime" appears to be a release version or very close to it. Files for the system, bootloader, boot partition, and modem are available to flash in the TWRP custom recovery, or you can try to simply flash the entire KDZ file in LG's proprietary Flash Tool. Naturally this isn't recommended for owners of any non-H815 versions of the G4, and even those who have the right model might want to wait for an official over-the-air update if they're not already rooted.
Early users report that this build is cosmetically identical to the older software, with LG's skin more or less intact and no hugely changed core features beyond the updates in AOSP. Unlocked phones in Europe and Asia should start seeing over-the-air updates later this week, with more specialized territory and carrier models following over the next few months.