I know the downgrading procedure using LGUP.
I tried downgrading Marshmallow firmware F500S20m and F500S20v to Lollipop F500SAT-00-V11i-SKT-KR-NOV-10-2015+0.tot, but the phone got stuck into bootloop after successful flashing and factory reset did not help.
Upgrading it back to Marshmallow...