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?
I upgrade Fedora from one version to another as doing a clean install would be a lot of work. Maybe I’m just too much of a rookie, but I don’t see the advantages of a clean install.
Even if I installed Fedora on a new computer, I’d just use my clonezilla backup if possible. But I haven’t tried it so I don’t know if this would work.