I have been trying to use various Linux distros for a long, long time, with some successes. But...

I keep hearing how easy it is to change desktops. And I've not managed it yet.

Right now I am running Ubuntu MATE 17.10. I though I might install KDE. It was suggested I open a terminal and type:

sudo apt install kubuntu-desktop

So I did. And it took quite a while to download and install a lot of files.

But when I rebooted, I'm still using MATE, or at least it looks as though I am.

Do I need to make a selection somewhere? Where? How permanent/temporary will that be?

I'm not an Ubuntu user, and I'm assuming you're using autologin (so no login display where you can choose the preferred desktop environment).

AFAIU, the choice of desktop environments (launched via .desktop files) are located in the directory /usr/share/xsessions/.

This might be relevant as well
https://www.linuxquestions.org/quest...nt-4175606299/

Log out.
Explore the display manager for an option for different desktops.

Newbie problem solved.

To build on what NewbProgrammer said, somewhere on the login screen there is an icon or menu item to allow you to switch between desktop environments. Sometimes it's near the login dialog, sometimes it's near the top or bottom of the screen. Click around and you will find it.

I thought so myself, but did not see anything but the ability to log in as myself or an un-created Other.

Is there an Ubuntu icon next to your name on the log-in screen? If so, clicking on that should give you a menu. If there isn't try any icons at the top of the screen.

maybe you heard about virtual desktops?
on most distros you can change them with the key combination Ctrl-Alt-left/right/up/down.

I saw the icon. It was ghosted, nothing I did to it worked. Until a couple of logouts and a reboot followed by another logout, then it worked. So, for me, this topic is closed. Sure wish it was easier to figure out, I'm sure more n00bs will have the same issue.