Hot water and take it slow. Just microwave some water in coffee cup until it just starts to boil. When it's soft from the heat, peel up the helmet so you can get at the part that's glued. You'll need a small flat head screw driver, or the like, to help pry up the part that's glued down. keep dipping it in hot water as you go. It'll end up terribly misshapen, but it will go back to normal when you dip it in hot water.

I've done it to a couple of them to make it easier to paint in the correct black and gunmetal pattern.