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?
Xp to 7 was upgrade. 7 to 10 was clean
11 to Mint 21.2 was, obviously, clean
The few times i tried linux i used ubuntu. And each time a newer iteration was published a complete wipe and format was done for the new one