"Logging out" of a certificate (i.e. getting the browser to ask you which certificate to use after you have chosen one already) can be a bit tricky with most current web browsers. Normally restarting your browser is the simplest way to do this. You can also achieve the same effect with different tricks depending on what browser you use:
Method : click 'History' menu (press ALT key to display menu if it's not there) → 'Clear Recent History' → tick only 'Active Logins' and click 'Clear Now' button. Then close this page and reload the previous page.