just referred to https://fedoraproject.org/wiki/Upgrading_Fedora_using_yum#Fedora_18_-.3E_Fedora_19_.28pre_release_branched.29
I would recommend that you try this in virtual environment , such as in VMware , Virtualbox and KVM etc. Because F19 is beta release as of now.
F18 is running within KVM.
on the F18 VM
[root@localhost ~]# cat /etc/fedora-release
Fedora release 18 (Spherical Cow)
[root@localhost ~]# rpm --import https://fedoraproject.org/static/FB4B18E6.txt
[root@localhost ~]# yum update yum
[root@localhost ~]# yum --releasever=19 distro-sync
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reboot the VM.
after booting the OS.
I was able to upgrade to Fedora 19 beta :D
[root@localhost ~]# cat /etc/fedora-release
Fedora release 19 (Schrödinger’s Cat)
[root@localhost ~]# uname -ri
3.9.4-300.fc19.x86_64 x86_64
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