Downgrade Android 6 Marshmallow version
It is possible by using TWRP or any other custom recovery of your choice but you must have desired firmware file as recovery flashable zip archive first. Unmounting​ adoptable storage and performing all available wipes from recovery before downgrading is also required. You can find detailed instructions on what custom recovery is and how to install it on specialized forums (I recommend XDA Developers one) as well, as desired firmware version for your downgrade (head to your device thread and seek there if you interested).