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 .
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 . The
Fedora Project wiki also contains instructions  on how to upgrade
from a previous release of Fedora to a version receiving updates.