Hi mate- you will need to uninstall your existing copy - either through windows add remove or via the cd, then you will be able to install the new version.
if you want to keep your old folders for campaigns , skins etc then copy the entire il2fb folder to another directory or better still another partition and name it something like il2backup.
for good info on installing/uninstalling, multiple installs and trouble shooting see here
http://www.airwarfare.com/guides/patching.htm
good luck