1 - Get a fresh copy of the installer from the Yahoo! site. make sure that it's compatible with your computer and OS.
2 - Run the installer. It will reinstall over the previous version, you don't need to un-install the previous version unless you are prompted to do so. In this case, you'll probably be offered an automatic un-install from whitin the installer.
3 - Check that your settings are correct, like user name and password.
Electro, December 2011