There is a limit set on how bit iso file can be uploaded. Since last two versions the limit has changed. So if you have an older version you won't be able to update to the newest one from the webgui.
Please go to MAINTENANCE -> miscellaneous and download a copy of your setup and configuration on you PC. After that you can reflash the USB memory stick with the new DSS V6 release and after that restore the config and setup. The data will stay untouched.