The quickest way to upgrade wordpress is via subversion. Assuming you had already installed your blog via subversion then you can upgrade using the following steps:
Backup your database
Since the wordpress db contains only MyISAM tables, you can backup with a simple file copy.
cd /var/lib/mysql
cp -Rp wordpress/ wordpressBACKUP
Switch to the latest subversion url
Just run the svn switch command. This will add, update and delete scripts as required to bring you to the latest version.
cd /path/to/your/blog
svn switch http://svn.automattic.com/wordpress/tags/2.5/
Run the upgrade script
Finally, run the wordpress upgrade script, which upgrades the database and anything else it might need to do. Simply visit http://www.yoursite.com/blog/wp-admin/upgrade.php