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 archive contains a UT4MOD installer. Extract it to any place you like, then run it to upgrade your Jailbreak installation. If you can't run UT4MOD files, either get the plain archive version of the patch, or use UT2Kx Regcheck to fix the file association.
Note that you will have to use the plain archive version if your copy of JB2004b was also installed from a plain archive. You can find out by opening Manifest.ini in your UT2004\System folder. If that doesn't mention Jailbreak, it's a plain archive installation. If it does mention Jailbreak, but the Version number is 100 or 101, you might want to use the full UT4MOD installer instead, as you only have JB2004 or JB2004a. (Version 102 is JB2004b, regardless of source.)