This one is a bit tricky and if you are not good with uninstalling programs like iTunes you will need to call Apple support for help.
iTunes consists of 5 parts that have to be removed from your programs in a certain order which they will walk you through and then do a reinstall.
Don't worry - You WILL NOT lose your iTunes library or anything else in it. After that you should be back up and running in no time with the latest version.