There are two scenarios that require you to change your password.
You forgot your password and can't log in in which case just select 'Forgot Password' on the log in page, you will receive a reset password link in your email then proceed to change your password.
You have logged in and you want to change your password for whatever reason ;). Here's how to do it :
On the account homepage, select My Account from the menu on the extreme left. You can fill out the various profile data or just scroll all the way down to the Change Password section and change your password
That's it. You've reset your password.