Is it coming up with a login screen first, or is it sending you straight to the desktop? If it sends you straight to the desktop, is there a "log out" option you can use that will then take you to a login screen?
If you can get to a login screen, there is often a drop-down menu somewhere to choose what desktop environment you want after logging in. If you don't see KDE in the drop-down menu options, that may mean that you need to install it first to be able to use it.
If this is a fresh install and you don't have much to lose yet, you can also go for the easiest approach and just install the version that has KDE as the default. You can get that at
https://robolinux.org/k-install/
