First, you need to login to the website with your existing username and password (see the What do I do if I forgot my password?" FAQ question if you forgot or lost your password).

To change your password:

1. Login to the website.
2. On the left side under the section titled with your username, there is a link named "my account". Click it.
3. Under that there will then be a link named "edit account". Click it.
4. On the page that comes up (titled "Edit account"), there will be an item named "Password:" with two edit fields under it. Enter your new password into both edit boxes (it must be the same in both).
5. Scroll down to the bottom of the page and click on the "Save user information" button.