Log in to My Account, select "Edit Profile" from the Account Settings menu. You can change your password from there.
If you have forgotten your password and is unable to log in:
Click the "Forgot Password" link on the login page. This will generate a temporary password, which will be sent to the email address registered with your My Account. You will then need to input this into the "Password" field of the login page. Please note this is case sensitive and will not work if spaces are left before or after the password.