each user picks his own DE (as long as it's installed). Just log your user out then on the log in (greeter) screen you should see an entry titles "sessions" on the bottom. Click that then check the DE of your choice. A window will then open asking if you want to use that DE just for this session or to make it your default. Check default. Presto, you are now a KDE head.
edit: in retrospect, I would assume the purpose for your post is you do not even have KDE installed. open a ternimal:
yum groupinstall "KDE (K Desktop Environment)"
then see above directions.