How do I resume a release upgrade?

Try these steps, from the command line:

Fix broken dependencies:

$ sudo apt-get install -f

If it still doesn't work, try this to fix/reconfigure the existing, unpacked-but-not-yet-configured packages:

$ sudo dpkg --configure -a

Then try the upgrade again.

If it fails again, you may want to delete all the already- downloaded packages (they should be stored in /var/cache/apt/archives), do

$ sudo apt-get clean

That will clean up the packages but if you're still stuck in some weird state, I'm not sure what else to suggest.

$ sudo apt-get autoremove -y

That will remove any now obsolete packages.

Of course, before you do anything you should back up any important contents of your /home/dave directory (or whatever your $HOME directory is).

It appears that sudo screen -D -r will reconnect to a disconnected install in a 12.04 to 16.04 upgrade. This means no killing of packages or anything like that. This assumes that you didn't actively stop the upgrade, just lost access to the session.

My 16.04 to 18.04 upgrade had interrupted as I had stopped the process by mistake. I guess the process was towards the end (installing wifi packages). Nothing of above worked and kept giving "dpkg fronend is locked by another process". A system reboot followed by

$ sudo dpkg --configure -a

resumed the process and completed upgrade to 18.04 successfully.