When you are at the login screen which I am posting a link to the official Fedora image you can look at. At the bottom there are:
"Language-Session-Restart-Shut Down", go to "Session" and you should see Gnome and KDE listed to choose from.
You can also install a "desktop switching tool" that will be accessed from "System>Preferences>More Preferences>Desktop Switching Tool" or from a terminal "$ switchdesk".
# yum install switchdesk
# yum install switchdesk-gui