update/upgrade will update you within a release. It will not take you to the next release.
Make sure that you have backed up every file you would like to keep and all settings, such as MAC address, etc. All files and settings are erased during this upgrade.
Follow this link and scroll to the bottom to find the link to the update. It is reply 9958
The file is currently dragonboard410c_sdcard_install_debian-36.zip .
If you have forgotten how to update the board using an SD card, look here