You must know the password of the Yahoomail account you desire to terminate. If you do not remember the password, you can recover or reset it by using the following Yahoo password helper link:
It is recommended to use a laptop or a desktop while deleting your account as one or more webpages that you may need to access while deleting the account may not load correctly on a mobile device.)
You can delete your Yahoomail account by following the steps below:
1.Click on this Yahoo link:
2.Enter the Yahoo ID and associated password of the Yahoomail account you desire to terminate.
3.Read the section "Before continuing, please consider the following information" thoroughly
4.Confirm your password in the field provided
5.Enter the CAPTCHA code correctly.
6.Click on "Terminate this Account". Your account will be closed permanently.
Hope this will resolve your issue.