This command will work
$ sudo do-release-upgrade -d
If it says that ‘no release found’ then you should launch `update-manager` and click on settings. Make sure you have `check for any release` instead of `Only long-term release` option selected. I might be wrong in exact wordings of options. In nutshell, open update-manger, click on settings and do the obvious. Reload, and you should be able to update it.
Also make sure that repository you are downloading from is in sync with main repository. Or what the hell, just use the main repository. For more detailed and step-by-step process. Check this link.