Upgrading to WordPress 2.6.5

Hi, just thought I would tap into all you lovely people to help me. New to this, we are using 2.6.3 and have been told to upgrade to 2.6.5. How do we do this? And more importantly, will we lose all our posts/pages?

On another matter, we want to track how many people visit the site, is there a plugin on Word Press for us to do this?

Thanks.