how do delete yahoo account?

Here two ways:

First way is not to use it for 4
months and Yahoo! will deactivate
it due to inactivity.

More inactivity Info:
http://help.yahoo.com/l/us/yahoo/mail/original/account/account-04.html

OR...
Click the link below for deleting your
Yahoo! Account .
https://edit.yahoo.com/config/delete_user

➊.-Sign in to the Yahoo Account
you wish to delete, using your
Sign In ID and Password

➋.-Scroll down to the bottom
of the new page,
and underneath the question:
➣"Are you sure you want to terminate
your Yahoo! Account?"
➣Type in your Password.
➣ Click the ....
"Yes Terminate This Account".
➣After the account has been deleted
you´ll not be able to use the ID you
have closed.

More Information click on
Yahoo! Help

http://help.yahoo.com/l/us/yahoo/mail/original/account/account-11.html