To me it sounds like you have a problem with the used resolution..if you use Gnome desktop (probably do), see the System menu, there Settings, there Resolution (or something close to those). If that won't help (for example you can't change resolution or the setting is not saved), you may have to edit /etc/X11/xorg.conf to set the resolution to that of your screen's native resolution.
To change Gnome to KDE, you need to install the KDE desktop (all KDE packages), then log out and from the login manager select you session to be KDE instead of Gnome. After this, if you want, you can remove Gnome. To install the new desktop, use your package management application found in the menu (probably System menu, altough it could be in Applications as well; I'm not that familiar with Fedora, but it is there); once it's started, search for kde (or kde-desktop) and start installing..
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