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. It works perfectly fine and when it doesn’t figuring out what’s going on learns me something and several times has resulted in fix commits to the packages.
E: there’s some people saying they do clean installs on Ubuntu. They’re right that ubuntu breaks shit all the time but I’ve solved that by simply not using the bad distros.