Upgrade instructions are included in the documentations folder of the downloaded code as well as a note if you want to see more specific changes that are done from version to version read the changelog.txt file. The main upgrade documentation states to upload all files from the download except config files and add any new config variables to your older configs and run the upgrade script and redo xml plugins if applicable for vb.
Basically reading the documentation from 8.4 to 8.42 you want to upload all the files except the 2 config files. You would add the new $cookie_salt variable to the config-inc.php file and set accordingly. You would then run the upgrade script and upgrade 8.4. You would then follow the instructions in the upgrade documentation to redo your vb xml plugins and install that into vbulletin.