Not sure if you know this or not - but all “update” does is sync to latest in apt database… it doesn’t upgrade any software (it’s confusing I know, because in Red Hat land “yum update” [and maybe even “dnf update”] does upgrade packages [and even kernel] to latest in repos).
Here’s how I upgraded to 20.04 (from 18.04.4)
“sudo apt update”
“sudo apt upgrade”
“ssh 0” (ssh to my localhost where “0” is shorthand for “127.0.0.1” - but it’s actually 0.0.0.0 - kinda the same thing) to find out if I need to reboot (it will tell you if you need to reboot) - note : I ALWAYS install “openssh-server” on desktops - one of the first packages I install - I don’t see why it can’t be default. Reboot if it tells you to…
“sudo apt dist-upgrade” (it may already be at latest)
“sudo do-release-upgrade -d” - can take hours (in my case? about 1-2 hours) - note the “-d” forces it to upgrade to 20.04
I’m kinda making the assumption you want to upgrade 18.04.x LTS to 20.04 LTS.
Above should also work going from 19.10 to 20.04… I don’t know about any intermediates (e.g. 18.10 or 19.04 - but they’re out of date and EOL, and you may have to change your /etc/apt/sources.list to use the “oldversion” repositories - can’t remember the exact URL).
I’ve done 3 so far, desktop upgrades from 18.04.4 to 20.04, these were relatively fresh installs of 18.04… and not encountered any issues…
