Just now I made the update from Fedora 17 to 18. I didnt know that fedora 18 has a new update util called fedUp. If you are interested you could read here: https://fedoraproject.org/wiki/FedUp
Just do the following
yum --enablerepo=updates-testing install fedup
fedup --network 18
It should ask for a reboot after some downloads, and that should just do the trick. atleast it did for me.