1) Cancelling your Yahoo account permanently is pretty simple. You first need to go to the Yahoo Account Delete page.
2) Once you go to the link mentioned above, you will be asked to Sign in using your existing id and password.
3) It is highly likely that you will be required to enter a CAPTCHA to make sure you are a human and not a bot.
4) Once you are signed in, you will see two options: Terminate This Account or Return to Account Information.
5) Click the Terminate This Account.
Congratz! You just deleted your Yahoo account.
But you can recover your account for next 90 days after deleting it. After 90 days, it will no longer be in Yahoo's database or servers. Gone!