clean install: you make a backup, nuke the computer, install a fresh upgraded copy of the distro you want from a live usb, copy your data again to the computer.
upgrade: you wait ‘till the distro’ developers release an upgrade you can directly install from your soon to be old distro, you use a command like sudo do-release-upgrade
and why do you upgrade like that?
It depends on how many versions I am away from the latest, and how much I’ve messed with the distro.
Usually I stay on an Ubuntu LTS and upgrade from LTS to LTS when that upgrade path is ready. I upgraded from 20.04 to 22.04 this way.
But this time I wanted Pipewire in 24.04, and didn’t want to wait for a 22.04 to 24.04 upgrade to be ready. I’m using a bluetooth headset and Pulseaudio is pretty terrible at switching headset profiles. Between not wanting to upgrade an upgraded install, and having messed with Pulseaudio quite a bit trying to get it working, I went ahead and clean installed 24.04 and moved some configs over.