1. Download the latest build from the members area.
2. Extract the archive to your hard disk (or to the server with any available utilities).
3. Upload (or move) all files EXCEPT for the cache_builders/custom, includes and styles directories.
4. Ensure chmods. All directories should be 777. All files should be 666.
5. After uploading (or moving) all directories, and ensuring all chmods are correct, run the install/upgrade.php script.
6. After you've gone through the upgrade procedure in the upgrade.php file, be sure to REMOVE the install and importers directories.
NOTE Please be sure to create a backup of your database and files before upgrading your installation; the upgrade should happen without a hitch, however, it's better to be safe than sorry...