Press * on the keypad, scroll to Home, and choose Options > Settings. Choose Master reset. Note that the contacts list on the device is not affected and that the sources you've chosen remain in effect, although all customization that is not synchronized with Yahoo! is erased.

The next time you start Yahoo! Go 3.0 after a reset, you must choose a country, accept the Yahoo! Terms of Service, and provide your Yahoo! ID and password to sign in.