Mine is stuck on Android 5.0 and hasn't had a security update since Nov 2015. It has never been tied to any network and is the international model D855. Anybody else with this mobile been upgraded? Getting very close to giving up on them and rooting it to load custom ROM.
Just google around and it appears that it's possible to upgrade to Marshmallow using LG's mobile support tool on the desktop. My wife's G2 also does not do OTA update unless using the support tool. You might want to give that a try.
Mixed review on Marshmallow on G3 though.