MXGP - The Official Motocross Videogame

There are three places where you can edit your helmet. the first of these is right at the beginning where you have to create your custom rider in order to continue. You can find this in Appearance and at the top is the helmet option, simply change the helmet once and confirm to unlock the achievement.

But if you rushed it and left everything to the default then don't worry, because you can also edit your rider in My MXGP in the main menu and in your Locker in the Career menu.