Update LG G3 (D855) to marshmallow by installing official CM 13 nightly for LG G3 (Unlocked) D855 .As of now first official cm13 nightly is released by cyanogenmod team based on latest android marshmallow 6.0.You can download cm13 official nightly for LG G3 D855 from official CyanogenMod website from the provided in the download section below.

As the rom is new and no one yet test it so we request your to test it out and
report in comment section.As this is first cm13 nightly there may be
bugs in it.I refer your to make backup of your current rom before flashing this rom.Lets start the guide on How to install Official Cm13 rom on LG G3 (Unlocked) D855.

See also:Check out with other devices is going to get official Cm13 rom

#How to install Official CM13 rom in  LG G3

1.Download official CM 13 nightly for LG G3.and the appropriate Gapps 6.0 package
from link given below Copy it in your in your device.
 
2.Now place the Official Cyanogenmod 13 rom and gapps 6.0 zip file in sd card.

3.Now reboot to Recovery mode.

4.Then flash CM13 rom and Gapps 6.0 from recovery in it.
 
5.Now Clear cache and Clear Data.
 
6.Reboot your device.Done! you just flash Official cyanogenmod 13 rom in your  L.G G3.

Enjoy Android Marshmallow 6.0 !.