I love WordPress and recommend it to all my clients. Today, the newest version was released. You can read about the fixes and download the upgrade here. Here's some tips on upgrading:
NOTE: Try to avoid ‘hacking' the core code on WordPress, it makes upgrading much easier. I do have a few ‘hacks' but I keep them documented so that when I download the latest version, I can make my edits and move on. Also avoid putting any type of custom files or folders in any folder outside your wp-content folder.
As long as you haven't hacked WordPress, the upgrade process is pretty straight-forward (Images are of Panic's Transmit 3.5.5)
1. Open your FTP Client, select all files from the WordPress upgrade but EXCLUDE the wp-content folder. Copy over existing folders and files.
2. Now, open the wp-content folder on your source and destination folder. Copy over the index.php file.
3. Lastly, rummage through the wp-content subfolders on your source and destination folder. Copy over themes and plugins as needed, avoiding deleting any of the plugins and themes that you've added and modified.
4. Your next step is simply to login to your Administrative interface (wp-admin). You'll be prompted to upgrade your database. Click a button and you're done!
There you have it, you're upgraded. Hope this helps you out!