STEP 1: Login to your Yahoo Mail. In the address line, you will see the name of the Yahoo Server which you are accessing.
the f 379 or 380 can be seen in the tool bar once u have logged in with ur username and password (Kindly have a close look at the tool bar whether it is f379, 380 or some number)
STEP 2: Now enter the following URL in the same window in which you have opened Yahoo Mail
Here, replace fXXX by your own Yahoo Mail Server. In the example above, the URL will be http://us.f380.mail.yahoo.com/dc/landing.
STEP 3: A page will open, just click on “Try It Now” button to upgrade your Yahoo Mail!
That’s it! You are done. If at any time, you need to come back to old version of Yahoo Mail, you can do so by clicking on “Switch Back” link at the top near “Logout” link, or through Yahoo Mail Options.