I really wanted to use new wordpress 2.5 version for this site since I’ve been testing RC1 release in my local. Because of new database changes, new refreshed admin panel, improved post editor (image uploading, gallery and full screen editing, etc.), ability to upgrade plugin automatically and much more in WordPress 2.5. Today, I just upgraded to WordPress 2.5 for my site and it was successful without having any problems. As my experience, I download the latest release from wordpress site, and I disabled all the plugins i’m using for my wordpres. After that I upload all the files from wordpress zip files. Then I run upgrade.php file from wp-admin folder. It upgrades automatically. Then I enable plugins again. Here are the simple steps you can upgrade wordpress 2.5 from ealier version.
Backup database first !!!
This is really important for your blog!. Backup your wordpress by using both exporting your wordpress method and downloading database files from your MySql database method.
Download the Latest Release
Download the latest release from WordPress site. And extract that zip file into your local machine.
Disable all the plugins
Go to the plugins page and mark the active plugins which are currently using for your blog. Then diactivate them.
Upload the WordPress files
Upload the WordPress files from your machine to the your site using FTP program.
type your site url and /wp-admin/upgrade.php . (example: http://www.example.com/wp-admin/upgrade.php). If you installed wordpress different from your home page, Use something like this (example: http://www.example.com/wordpress/wp-admin/upgrade.php)
After successful upgrading, it will redirect to new interface login page.
Reactivate your plugins
You disabled your plugins before you upgrade. So you must activate them again inorder to run your wordpress without any problems.