It's fairly simple, put the helmet in the closed position (chin bar down). Flip up the shield fully open. Press the black button in the center of the mount to release the mechanism, then pull the shield sorta up and out.
It is quite hard to do the first few times. I may seem you are going to break something, but it won't (if you are doing it right).
To put it back, you reverse this. Make sure you have the ratchet part on the bottom lined up correctly.
It's fairly simple to do, but requires a lot of force to do it. And, practice it a lot. The first few times you do it, it's very hard. It requires a lot more force than most of us would like to use on plastic parts... but, I've never broken anything, despite feeling like I was about to...
I just changed the face shield on my nolan 102; I could not do it until I went to google and searched for instructions on how to do this. Once you have the instructions, it is pretty easy. there were photos showing how to do it.
just type in something like: how to change face shield on nolan n102 helmet-----or something similar.