You can screw down the upper parts of the keyboard, with the keys, joystick.
And you can wash with lukewarm water

soap or other not agressive cleaning things can be used. (Without break the warranty label)
If many stuff between the keys, you can easy remove all keys from their places, but be carefully: do not break the fixing bows!
The joystick is can be easy disassembled.
After all keys removed, you can wash anything.
When clean, lets dry it, and reassemble. Keyboard layout photo good idea for find the place of all keys

Good idea to use silicone spray (can be bought in car shops for a plastic care), blow the downside of the keyboard top, to make lubrication for the keys. After this the keys can moving a very fine.