Like SScorpio said, provided your mister is connected to internet (a wifi dongle is enough for that) just run the default downloader tool or use the update_all script : you can get it here, just slap it in your /scripts directory and run it from the interface or from the shell.If you were to get one now, how does it work when new systems are added? Do you have to completely change any case you already have, for example? Do you add new physical components? Or is it just a matter of uploading some new files to the thing you already have?
Update_all isn't bundled with the default distribution because it download additional stuff (like beta cores, bios or mame roms) compared to the default tool.