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.
Optimus G was the first device in the flagship G series and it was launched a long time back. So it is quite obvious that official support for this device is no longer available. But Android being open source, there are always custom ROMs to bail you out. CyanogenMod teams have been working to provide CM 12.1 ROMs for almost all of the phones. LG Optimus G can be upgraded to Android 5.1.1 using CM 12.1. Follow this guide.