Comment 4 for bug 1594233

Before trying the commands above, you should remove any third-party or PPA packages from your computer. Go to Software & Updates -> Other Software to find what PPAs you have installed. Then use `sudo apt install ppa-purge ; sudo ppa-purge ppa_name` (replacing 'ppa_name' with the relevant PPA, typing in multiple PPAs separated by spaces if necessary). Then perhaps run `sudo apt update ; sudo apt upgrade` to ensure everything is fully up-to-date. You're also going to have to manually remove any software you've installed from DEB packages that aren't from the repositories. Use `sudo apt package_name` (replacing 'package_name' with the name of the package) to remove these manually (yes, you'll have to remember which packages you've installed manually).

After you've done all this, first try upgrading using the Software Update program (as you were trying originally). If that still doesn't work, try the commands I suggested in comment 3.