How can you upgrade
With earlier release i.e WordPress 2.7, upgrade process had been made quite easy, all you have to do is to go to Tools > Upgrade and click Upgrade Automatically and you’re done. It will take few minutes to download and install the new version to your server.
Alternatively if you have an older version than 2.7 or there is some problem with upgrading automatically, than it is better to upgrade manually. Download the latest version from here & extract it. Open your ftp client and connect to your hosting server. Overwrite all WordPress files with new ones. That’s it.