LG Optimus G is LG’s late 2012 flagship phone. It is powered by quad-core Qualcomm Snapdragon S4 Pro processor. The speedy processor is backed by 2 GB of RAM, 32 GB of ROM, a 4.7″ “True HD-IPS+” panel, LTE connectivity, a 2100 mAh battery, and an 8 or 13 MP camera (depending on region). It originally came with ICS OS but it was upgradable to Jelly Bean.
LG Optimus G is comparatively an older device so it wont be getting Android 6.0 support. However there are custom ROMs available using which you can update LG Optimus G to Android 6.0. AOSP team have created an early build of Android 6.0 Marshmallow ROM. If you want to install it, follow the guide below.