Community > My Account > Iris Online Account > (If you've not made an In-game account click "Create New Iris Account) To change your pin Update Iris Password or Pin > Login with your ingame account login > Put new pin into the Change Security Pin box > Then click change.
That should be it.

GM too hungry eh XD ate the "Community" button. lol jk

What?

Since the site update you can no longer change your pin and it will be removed from the game soon.

If you want to change your password, log into the website with your game account username and password and then go to account settings in your profile.