1.In Settings choose Mail, Contacts, Calendars.
2.Enter to Add Account and choose Yahoo!.
3.Fill your name or your account name in Name.
4.Fill your Yahoo! Mail address—for example: email@example.com—in Address.
5.Fill the password for your Yahoo! Mail address.
6.Provide a description of the account.
7.Confirm the settings.
Based on following Yahoo! help thread: