I want know more about the new feature Face ID in iPhone X, if user hold it's iPhone X upside down then Face ID will work?
Not on iPhone X/XR/XS
Face ID, as currently implemented (2017), does not work in landscape orientation. (The camera system is optimized for portrait.)
From Rene Ritchie's iPhone X review.
However the new 2018 iPad Pro devices do support Face ID in any orientation.