How to clean air-o-swiss humidifier?

Gererally according to the booklet in the box.
If you no longer have those instructions, simply clean out the
basin, make sure no mold, dust, hardwater or calcium is deposited.
Use a 10% bleach water solution to kill bacteria, and thats is basically it.
Just dont miss any spots, but humdifiers should be cleaned every few uses.

This both shows how to clean the unit as well as how to reset the cleaning light if it has come on:

Per the Air-O-Swiss FAQ:

Q: Cleaning light is flashing (AOS U650, AOS 7135, AOS 7142, AOS 7144, AOS 7147)
A: Switch the unit OFF and unplug it. Empty the water in the base. Rinse the base with hot/clean water. Do not submerge the base! Visually inspect the base for dirt or mineral build up. If soft water is used decalcifying the unit every 4 weeks with EZ Cal is suggested. If hard water is used the unit should be decalcified every 2 weeks. Do not forget to reset the clean light after resetting the unit.

Q: Reset cleaning light (AOS U650, AOS 7135, AOS 7142, AOS 7144, AOS 7147)
A: The cleaning light does not automatically reset. Hold down the power button (button A in instruction manual) until the light stops flashing. The light automatically comes on approximately 14 days after plugging in the unit.