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.

Use tasksel to install, then choose a different desktop on the login screen