This patch will upgrade JB2004b, either installed from the web or from the second disc of UT2004 Editor's Choice Edition, to the latest version. If, for some reason, you still have the initial release (JB2004 without "a" or "b") or JB2004a (SP1) installed, you will have to download the full installer package instead.
This download is a plain 7zip archive. To install it, simply extract its content to the UT2004 base folder, maintaining the directory structure and replacing any conflicting files. The archive does not contain any configuration files, so you don't risk losing any settings.