If you already have a DE installed (in your case KDE) you would do something like sudo apt install xfce and then restart (I don't think you actually have to, but I like to reboot after installing a DE). From there you should see a button on your login manager (SDDM for KDE I think?) That lets you pick the DE for the session. Usually looks like a drop down menu.
I mean people already answered your question, but I want to know why you want to leave kde plasma? It was the best I have found (after gnome, but in gnome I can't change shit)
Not OP, but KDE has always seemed super fidgety to me. Like, I just can't leave it alone, and get overwhelmed by all the fine grained tuning. I'm also not a fan of the plasticky design language.