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?
Upgrade. Don’t wanna mess with restoring/preserving data and configs.
It depends. For Fedora I just do a in place upgrade. However once in a blue moon I do a reinstall.
I upgrade in place more than once a week ( rolling release ).
Xp to 7 was upgrade. 7 to 10 was clean
make a backup
Pffftt… coward.
/s