As of 14th January 2014, Fedora 18 has reached its end of life for updates and support. No further updates, including security updates, will be available for Fedora 18. A previous reminder was sent on December 18th .
Fedora 19 will continue to receive updates until approximately one month after the release of Fedora 21. The maintenance schedule of Fedora releases is documented on the Fedora Project wiki . The Fedora Project wiki also contains instructions  on how to upgrade from a previous release of Fedora to a version receiving updates.