Changing your password is very simple.
First, go to your account management interface.
Click on the "My account" tab and then on the "Edit" option under "My password".
For security reasons, you must enter your current password before you can modify it. Players who have certified their accounts must also provide their secret answer.
All you have to do now is enter a new password.
Remember to choose something that's hard to guess, and don't share it with anyone!