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?
Fedora, I usually wait 1-2 weeks for the last bugs to be found+fixed and extensions to catch up, and then just upgrade in-place. Haven’t had a major upgrade problem for years now, it’s mostly as smooth as any other offline update. And I don’t feel like I have to reinstall the OS every few years on Linux either.