Cleaning a Joan device is as simple as using it.
Merely wipe the device with a soft, moistened cloth or paper towel. If you wish, you can use a screen cleaning solution, but be careful when using it for a Joan 6, so that the solution does not go underneath the casing, as it might permanently damage the device.
Also, make sure to unplug the device from any power source and not to allow the liquid to run or drip near the USB port(s) of the device.