[Q] Update Boot/firmware to Marshmallow

Hi,

My M9 is bootloader unlocked and S-ON, it's running ARHD12.0 which is lollipop.
I want to use ARHD20.0 which is Marshmallow.

What is the best way to update boot or firmware to support Marshmallow?

I am on TWRP 2.8.7 but I can't run TWRP 3.0.x coz it need 6.0 support as well (I think).

Thanks in advance for any help and suggestion.:D

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