You don't even *have* to do a complete uninstall, in order so that you can reinstall. If you just install the full GSX installer, it will be enough.
However, if you want to know anyway, just uninstall using the normal uninstaller from the Add/remove Programs section of the Windows Control Panel, like any other Windows program out there.
Removing these folders:
%APPDATA%\Virtuali
Will also trash all your GSX personal preference (customized airplanes and sceneries), but it's not required.