Here's how:

Click Options in the upper-right corner of the page and select Mail Options. In the upper-right of the page that opens, look for the message "The new Yahoo! Mail has arrived! Try it now! ". Click "Try it Now! " to get to the new Yahoo! Mail.

To come back to Yahoo! Mail Classic, click Switch Back under "Welcome, yourname" at the top of your Yahoo! Mail page.

Also, here's where to look if you're curious about your new version of Yahoo! Mail Classic, and whether you can switch back to the old one.