sadly most printers these days are not built to be opened, they are likely rivited or use some other non-easily repared method of closing them. You could try contacting HP, which may take forever, or possibly looking for a seem in the plastic that you could cut or wedge apart. (I do not recommend this, unless you are going to trash the printer if this problem is not resolved)
The dust is likely held on by static (formed by the sweeping motion of the scanner)
*Another Approach: you could try using a fur cloth and rub it to form static, then try a sweeping motion of your own to move the dust to a single end or corner of the printer.
As i do not own this particular printer, this is the best i can do. If i did, i'd probably have a pile of peices on my floor trying to exlpain how to open it, while figuring out how to put it back together, lol