If you haven't registered already, you can find out how here. If you are registered, but want to upgrade to a more recent version, you should be able to do so by downloading the current trial release from the site and installing it over your existing installation. There is not normally any need to register again, nor do you need to re-enter your details.
Note that free upgrades are only available to registered users of the internet edition. They are not available for the retail (CD) version.