Tassilo Horn has a (very) short post on how he updates all his Emacs packages from the command line. It involves installing auto-package-update but after that it’s simply a matter of calling Emacs and running auto-package-update-now. It would be easy, for example, to set a cron job to update your packages at night when you’re not using the system.
His post is definitely worth a look if you have packages installed and like to update them regularly.