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?
Wait for the distro to officially release an upgrade path. Only do a fresh install if it doesn’t work.
On Windows however whenever I would get a new pc in which I was prepping for staff(I worked in IT) the first thing I’d do after unboxing it is a wipe of the factory Windows install and do a clean install with the latest ISO from Microsoft.
No bloatware, network managers, anti virus etc nonsense. We had all of our own stuff for that which applied via Group Policy anyway.