To set YUM to update automatically in RedHat flavor OSs execute the following set of commands. This will update your OS, install an application named "yum-cron" and set it to load when the machine is booted.

# yum update -y
# yum install yum-cron
# chkconfig yum-cron on

Now your system will automatically update daily if new software is available from the repositories listed in your /etc/yum.repos.d/ directory.