The format of Yahoo is not something you store locally on your computer and need to periodically update which means when Yahoo updates and you sign in using your username and password, you get the latest version.
It also means the "old one" no longer exists to go back to.
Many people, including myself, don't like change, but they eventually adapt and accept, only to complain at the next inevitable change.
At the top right of the page is a gear symbol, when you hover over it there is a help page. Might as well learn it. Your choices are 1. Use it. 2. Don't.