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 follow the official upgrade method. Can’t be bothered to mess around with anything more complicated than that. Besides, the devs probably understand the system better than I do, so there has to be a reason why that is the preferred way.