If you want to change your password/nickname/email or delete account, you can do it all in user center.
How to change email address?
Go to Profile page and click change email, if you change the email the previous account will not exist but the premium privileges will be transferred to new account.
How to delete account?
Go to Account Info page and click Delete Account at the bottom and follow the instructions. The account can't be restored, but you can sign up again with the same email.