Hi. I'm Smitty. I am a professional appliance repair consultant here to help you diagnose, and repair, your appliance.
The best thing to do is use a vacuum to get the dust out. For safery reasons it is not recommended that you remove the outer cover of the unit. There are componants in the unit that can cause great bodily harm if you are not experienced working near them.
You would also have to remove the unit from the window.
Please let me know if you have any other questions.