How to intstall UCI on Cowon S9
How to install a theme:
Make sure the theme you want to install is named correctly (mainmenu1.swf/mainmenu2.swf/music.swf and so on) and that your S9's USB-Mode is set into MSC (-> Settings/system/USB-Mode). Then open your S9 on your pc and drag and drop the theme you want into the S9's system/flashUI folder. (note that some themes come with extra applications, backgrounds etc.; Make sure you drop them in the same folder as well) . That's it. Unplug your S9 again (make sure you use safely remove device) and fire up your S9. the theme you installed should show up on the screen it was made for.
Taken somewhere from http://www.anythingbutipod.com/forum/
A list of UCI themes for Cowon S9 can be found here.