The upgrade instructions are fairly clear
that really depends on where you read, unfortuntely. Case in point:
https://codex.www.ads-software.com/Upgrading_WordPress
Step 1: Replace WordPress files
1. Get the latest WordPress. Either download and extract it to your computer or download it directly to the server.
1. As a reminder, to extract a tar.gz to a folder use this command, replacing (folder name) with the name of your folder: tar -xvzf latest.tar.gz -C ./(folder name)
2. Delete your old wp-includes and wp-admin directories.
3. Copy the new WordPress files to your server, overwriting old files in the root, except perhaps the wp-content folder (see “NOTE” below). You may use FTP or shell commands to do so. Note that this means *all* the files, including all the files in the root directory as well. If you use the default or classic theme and have customized it, then you can skip that theme.
NOTE The wp-content folder requires special handling, as do the plugins and themes folders. You should copy over the contents of these folders, not the entire folder. In some cases, copying the entire folder may overwrite all your customizations and added content.
Also take care to preserve the content of the wp-config.php file in the root directory. This file contains current settings for your existing installation, e.g. database sign-in information. Occasionally new versions of WordPress add statements to this file. (E.g. in version 2.5 the SECRET_KEY variable was added, see Extended upgrade instructions). Compare your existing file with the new installation file which is named wp-config-sample.php. Either transfer your settings to the sample-file and rename it to wp-config.php or copy the new statements from the sample file into your current file.
If those instructions are wrong then I’d welcome correct guidance about how to upgrade from WP 2.3.3.
you delete the core wordpress files.. core files being whats inside wp-includes/ and wp-admin/ and the wp-* that live in the root of your install, ie wp-settings.php, etc.. NOT to include your wp-config.php
you upload the new files.
you load the upgrade.php url in your browser.