Manual Update
These are the short instructions, if you want more check out the extended upgrade instructions. If you experience problems with the Three Step Update, you may want to review the more detailed upgrade instructions
For these instructions, it is assumed that your blog’s URL is http://example.com/wordpress/.
Step 1: Replace WordPress files
1. Get the latest WordPress zip (or tar.gz) file.
2. Unpack the zip file that you downloaded.
3. Delete the old wp-includes and wp-admin directories on your web host (through your FTP or shell access).
4. Using FTP or your shell access, upload the new WordPress files to your web host, overwriting old files.
NOTE – you should replace all the old WordPress files with the new ones, including those in the root directory like index.php, wp-login.php and so on. Simply copying the contents of the zip you downloaded over the top of your existing files should achieve this. Make sure you copy all the sub-directories as well. Don’t worry – your wp-config.php will be safe.
Be careful when you come to copying the wp-content directory. You should make sure that you only copy the files from this directory, rather than replacing your existing wp-content directory. This is where your themes and plugins live, so you will want to keep them. If you have customized the default or classic themes without renaming them, make sure not to overwrite those files, otherwise you will lose your changes. (Though you might want to compare them for new features or fixes..)
Lastly you should take a look at the wp-config-sample.php file, to see if any new settings have been introduced that you might want to add to your own wp-config.php.
http://codex.wordpress.org/Updating_WordPress
