you have to download something called EverCraft
http://ingame.ingame.de/filebase/index.php?action=file&cid=445&fid=2632here is a download link.
it's a tool to automaticaly switch versions of WC3 , i dont rly know how to explain , there must be a readme.txt file in the zip.
you can also move you wc3 folder (rename it) , reinstall WC3 and go to the FTP of blizzard and download the patches (search on google : ftp blizzard warcraft patch , first link [thats how i did when i needed a patch])
pay me w/e you want i dont rly care ^^