In order to do this, run as the root user (or become root via su -) from a terminal window. On a fresh install of Fedora 20 with default options the yum-cron RPM is not installed, the first command below installs this RPM.
yum install -y yum-cron
su -c "gedit /etc/yum/yum-cron.conf"
and enter your password. After, change the line
apply_updates = no
apply_updates = yes
Save the file. You are now done. Yum-cron updates your system every time when there are new updates available.