I assume you wish to access email in a different Yahoo! Mail account. Yahoo! allows you to access only one account at a time. To sign in as a different user, you must first sign out from the account you are currently accessing.
To sign out from Yahoo! Mail, click on the "Sign out" link in the upper left corner of the web page:
Then go to the Yahoo! Mail page (http://mail.yahoo.com), and sign in with the new user ID and password.
See also this Yahoo! help pages:
http://help.yahoo.com/l/us/yahoo/mail/ymail/account/account-13.html
Hope that's helpful.
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