I have tried changing my last name in every conceivable place possible on Yahoo and it still shows my old last name when I send an email ?
Hover over the far right gear icon ⚙ in Yahoo mail and then click "settings". Then click "accounts" in left column. Select the primary address. On the next page you can do the following: Change FROM name and REPLY TO address FROM name can be blank and then only the email address will appear. REPLY TO can be blank and then the FROM address will be used.