WordPress 2.7 is now the latest version of WordPress and it comes with many new features. Below is the step by step guide line to upgrade your wordpress blog.
 Backup your entire blog (Web contents as well as your database).
 Rename wp-config-smple.php to wp-config.php and made the required changes in it.
 Upload your new worpress files and overwrite existing files.
 Now browse http://yourdomain.com/wp-admin/upgrade.php
You are done now.
Note: You can also request us to upgrade your blog on your behalf. Refer this post