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’m using a rolling release at the moment, but when I used a more stable release, I always did the upgrade (following the official instructions) because it’s faster and more convenient.
I learned the hard way to always keep a backup of my important stuff, regardless of the OS.
The only time I redid a clean install was when I accidentally fucked up my entire filesystem’s permissions.