You'll need to go 3rd-party for that. That device was released with 4.3, following the "2 major platform updates" that Google promises for its own devices it would have stopped at Lollipop (5.0). The spec page I'm looking at is not indicating it received any updates but that's not always accurate.
As for going 3rd-party, I suggest doing some research on the process. Do a Google search for 'Huawei Ascend G6 Marshmallow' and read everything, plus research what might be your best option for the ROM.
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