From Fedora Project Wiki

6. Updating Your System with yum

Use the update option to upgrade all of your Fedora system software to the latest version with one operation.

To perform a full system update, type this command:

su -c 'yum update'

At the prompt, enter the root password.

6.1. Automatically Updating Your System

The yum package supplied with Fedora includes scripts to perform full system updates every day. To activate automatic daily updates, enter this command:

su -c '/sbin/chkconfig --level 345 yum on; /sbin/service yum start'

At the prompt, enter the password for the root account.

How Daily Updates are Run
There is no separate yum service that runs on your system. The command given above enables the control script /etc/rc.d/init.d/yum. This control script activates the script /etc/cron.daily/yum.cron, which causes the cron service to automatically begin a system update at 4am each day.
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