How can I log out using Form.Authentication ?

The SignOut method removes the forms-authentication ticket information from the cookie or the URL if CookiesSupported is false. You can use the SignOut method in conjunction with the RedirectToLoginPage method to log one user out and allow a different user to log in.
If you run exclusively in cookieless mode, or if you support both authenticated and anonymous users, you should explicitly control the redirect to the login page if you require special business logic to execute as a result of removing the anonymous identifier.
When the SignOut method is called, a redirect to the application's login page is made by calling the Redirect method with the endResponse parameter set to false. The redirect does not take place until the current page has finished executing, so additional code can be run. If the code does not contain an explicit redirect to another page, the user is redirected to the login page configured in the application's configuration file.