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?
Neither. I use a rolling release distro.
But if I have to use release based distros, I probably would clean install.
A rolling release distro is basically a requirement for me. I abhor major release upgrades. They're usually labor intensive and often break things.