It looks like LG will not be getting the Android Marshmallow update. LG never officially confirm it but looking at how old the LG G2 is, it is safe to assume that LG will not be releasing the Marshmallow update for the LG G2.
While LG G2 will not be getting the Marshmallow update officially, they can still try out the new Android 6.0 Marshmallow using the custom ROM that the developers have come out with. There is currently an Android 6.0 Marshmallow custom ROM by CyanogenMod that users can try out. Not all models will work with the ROM so check if your device is on the list before you proceed.
Some LG G2 users got all excited when it was reported that a Verizon LG G2 running on the Android Marshmallow was spotted getting benchmarked. While the test does prove that the LG G2 is powerful enough to support the new OS, that does not mean that the OS is actually coming. It could just be anybody doing the test on their own.