Error when updating server to 17.04
If do-release-upgrade fails, you might need to edit the release-updates file. Open that file with a text editor (e.g. nano)
nano /etc/update-manager/release-upgrades
Edit the last line to say:
Prompt=normal
Then run do-release-upgrade (without the -d flag)
When the upgrade is complete, edit that line again to say
Prompt=lts