Login to logout

I am having a " Login " as an hyperlink in my master page. I want the login hyperlink text to change as logout when i logined the page i get the job recruiter home page or the job seeker page so on that the masterpage's Login should change to logout and for that i am using the session  variable. i want the session to be null when the text of hyperlink is Login and to destroy the session as soon as clicked the Logout.