how to upgrade from unsupported Ubuntu version 15.04 to more recent Ubuntu version -- simply changing sources to old-releases.ubuntu.com not working
1520
How to install software or upgrade from an old unsupported release?
113056
How to upgrade from 12.04 to 12.10 using CD?
151605
upgrading from 15.10 to 16.04 apt not installed
123413
Will apt-get dist-upgrade upgrade my system to a newer version?
2236
Why don't the Ubuntu repositories have the latest versions of software?
74079
How to Upgrade Ubuntu 12.04.2, 12.04.3 to 12.04.4, the next Hardware Enablement Stack?
42634
How do I upgrade to the development release (aka. Ubuntu+1)?
31506
How do I update Ubuntu manually?
19395
How can I install more recent versions of software than what Debian provides?
132933
Is there a tool to update my PPA sources.list entries after upgrading?
35093
Upgrading from the command line?
23096
How to remove obsolete packages after failed release upgrade via do-release-upgrade on Ubuntu?
5820
How to fix the “Unable to calculate upgrade” issue from command line when upgrading from 12.04 to 14.04?
41519
What can I do if a package is not available on the newest version of Ubuntu (18.04) but is available on older versions?
115421
Ubuntu 13.10 to 14.04 Lts does not upgrade, keeps showing error
100494
Update Manager asking me to update kernel but apt-get upgrade does not?
58979
How can I upgrade Ubuntu LTS to the next one offline?