go to the http://www.wordpress.org main page for the download link to version 2.3.1
on the same page, you can also enter "upgrade" into the search field, and read the results
always a good idea, definitely back everything up before proceeding
contrary to what you'll read in the "upgrade" threads, it seems to be the recommended consensus here to delete your original WP installed files and do a fresh install of the newer version of WP, rather than overwriting the old folders and files
also contrary to what you may read, don't run "upgrade.php" or "install.php" after you FTP the new files into place
you will need to move wp-config.php from your backup to the new install
I don't have any personal experience, but it's my understanding if you have any plug-ins activated, you'll need to reinstall them; also they may or may not work with the new version of WP; look for additional advice on this subject