It can be a pain in the rear end (as in, the process has failed on occasion, though somewhat rare), but the instructions are found here: https://www.att.com/esupport
Cancel your Yahoo Small Business account May 24, 2019 Delete Your AT&T Email Account - Email Support When you cancel an AT&T service, the AT&T email address, also known as a Member ID, associated with that account isn't removed. You can continue to use that email address as an AT&T free email account without any charge. If you’d like to delete the email address, you must do that manually.
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Mar 18, 2015 Help for your Yahoo Account Set up, use, and manage Yahoo Account Key to sign in without a password Help for your Yahoo Account