This is a warning to all those using Wordpress. Latest WordPress 2.3.1 Apparently Vulnerable To Hackers
You should be able to just upload and write the new files to the respective folders. It shouldn't be a problem. Do a backup if you're nervous. Ask the folks at wordpress.org if you need help with it.
Yeah, do a backup of the database and all files on your server. Delete all the wordpress files on your server (minus your theme) and copy the new files over. You then will see a link that says "Database needs updated" when you go to your site (or maybe you have to use a specific link, I forget) and you click OK and it updates your database to work with 2.3.1. You'll probably have to copy over your wordpress config file as well (prior to clicking the database ugrade link) Glad I haven't updated all my sites yet!