If you want to change the actual UI on your Nintendo Switch console, please note this typically requires a running custom firmware (like Atmosphere):
While you can't download the UI separately, the system software—which includes the UI—receives regular updates from Nintendo. These updates often add new features, improve the system's stability, and enhance security, which can affect the look and feel of your experience. nintendo switch ui download link
This is the essential homebrew application used to safely install, organize, and manage UI themes on your Switch. Step-by-Step Guide to Customizing Your Switch UI If you want to change the actual UI