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?
"clean install" is Windows-user logic. Doesn't apply to Linux.
I feel like that may be true nowadays, but I remember back when I used to use ubuntu that the upgrade from 16.04 to 18.04 was pretty bad. Fedora has always worked great for me, but these days I only use rolling release distros in which case there aren't any major version updates in the first place, so the problem largely doesn't exist in the same context.