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?
Man just when I thought I got the hang of NixOS and setup everything already thanks to the new wiki. I dont think this is worth the trouble for me right now, but maybe in the future.