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?
upgrades have been working fine here, both linux and windows, for well over a decade.
only if a system is also being repurposed at the time of the ‘upgrade’, or if i’m changing the connection type of the boot drive (such as from sata to nvme, or switching an older system to ahci mode) do i install ‘from scratch’.