sudo crontab -e

You will need to use sudo because of the super-user privileges so cron is setup as root.

0 0 * * * aptitude -y update && aptitude -y upgrade && aptitude -y dist-upgrade && aptitude -y autoclean

The line above is the command that will run at 2:00 AM.   The 0 2 * * * signifies the time it will run, the format is a s follows “minute – hour – day of month – day of week”.  It will upgrade automatically at the time specified.  The upgrades include regular updates, kernel updates, security updates, so on and so forth.  The autoclean command removes old packages after they are no longer needed.   You can also just run the command yourself if you do not want cron to take care of it.

sudo aptitude -y update && sudo aptitude -y upgrade && sudo aptitude -y dist-upgrade && sudo aptitude autoclean

Note:  The -y will over write .conf files so if you have custom files make sure to have a backup of your .conf files before you run this manually.  Also you can cron a backup of your .conf files before cron runs the update.