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?
It depends on the distro. Some of them have some shitty ass upgrade process and it breaks shit, and others are just awesome. I personally use a rolling release so I don’t have to worry about upgrades. I do get some issues here and there with some big upgrades, but nothing really major. I’ve only had to reinstall twice in the last 2.5 years.