please

It isn't possible to change it back. Starting January 31st, 2019, the classic version of Yahoo Mail that you’re using on your computer will no longer be available. To ensure continued access to your Yahoo Mail account, you will be automatically upgraded to the latest version of Yahoo Mail or Basic Mail, depending on the compatibility of your computer’s web browser.

Classic is no more, gone since Jan 31 this year.

you can not , yahoo is out with the old in with the new , do not like the new ? move to gmail , fast easy to use