That depends on what you're trying to clean. Are you simply trying to make the system itself look nice? You can use the cleaning cloth included with the various screen protector kits for the Nintendo Wii U GamePad, the Nintendo DSi, and the Nintendo 3DS. This will allow you to gently remove dust from the system, and if need be, gently remove fingerprints as well.
If you're trying to clean the lens of the system though, you'll need to order the Nintendo Wii U Lens Cleaning Kit from Nintendo directly. It's available for purchase in their online store.