As of the 1st of December 2015, Fedora 21 has reached its end of life for updates and support. No further updates, including security updates, will be available for Fedora 21. A previous reminder was sent on 27th of May [0].
Fedora 22 will continue to receive updates until approximately one month after the release of Fedora 24. The maintenance schedule of Fedora releases is documented on the Fedora Project wiki [1]. The Fedora Project wiki also contains instructions [2] on how to upgrade from a previous release of Fedora to a version receiving updates.
Dennis
[0] https://lists.fedoraproject.org/archives/list/announce%40lists.fedoraproject... [1] https://fedoraproject.org/wiki/Fedora_Release_Life_Cycle#Maintenance_Schedul... [2] http://fedoraproject.org/wiki/DistributionUpgrades