Why can't I update my LG G3 D855 to Marshmallow or Lollipop. I'm stuck on Kitkat

I bought an LG G3 D855(International version) online and I can't update it to either Lollipop or Marshmallow. When I try to update it on the phone, it says that I am using the latest software version for current OS. I don't know what to do because I really want the latest software but I can't update it.
Does anybody know how I can update it?
Thank You

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