How to install 6.0 Marshmallow on D855 v21a

I'm trying to understand how can I install Marshmallow and root my v21a version but I don't get It at all. All what I found is about v30b or something similar, so, Is there someone who would like to explain me what I need to download and install because I don't want something to go wrong with my phone. Also I don't understand differences between global and europe versions, I live in Serbia (Europe) but my phone is v21a-TWN-XX (twn means Taiwan I think, I searched on google)

My phone:

Android version
5.0

Android security patch level
2015-11-01

Baseband version
MPSS.DI.2.0.1.c1.13.2-00002-M8974AAAAANPZM-1

Kernel version
3.4.0

Build number
LRX21R.A1447064049

Software version
V21a-TWN-XX