I made this guide because this is the second time i've heard someone accidentaly delete their account because they want to add a new account. For the sake of god, please don't touch any of the menus if you don't know what you're doing.
As always, pic unrelated.
There's a few steps you need to do to create a new account, the steps are fairly simple, but if you mess up, you might accidentally delete an account. Once lost, you won't be able to retrieve your account ever again. I'm not sure mailing GM will do any good, since you're deleting your account on your own accord. You have been warned that once it's deleted, it's impossible to retrieve it back (ゲームアカウントを削除後に、復旧等を行うことはできませんので、あらかじめご了承ください。) You can try, but the chance are next to none.
Anyway, move on to the "how to" part of this guide:
Login like usual, on the bottom of the login sidebar, there's a button called "game account tool", click on that one.
That's all. Once again I remind you, if you don't know what you're doing, please don't do. You might make yourself on the risk of account deletion, or account ban, whichever, which is not good in any way.