Early methods of upgrading Fedora, doesn’t work or aren’t supported anymore. The upgrade should be done with FedUp.
First of all users should backup their stuff and read Upgrade Issues, then let the magic begin:
1 – Do a full system upgrade
yum clean all; yum update; reboot
2 – Install latest release of FedUp
yum --enablerepo=updates-testing install fedup
3 – After that, reboot and choose System Upgrade from your bootloader.
When FedUp ends, Grub2 will show a new entry for Fedora 18.