There are 4 screws at the bottom  in the corners  - remove those and the top of the scanner will be separated.

This will not be an easy job to explain, since there are so many variences in the way the scanner bed attaches, and the position and number of screws that hold it together. Every plastic fastener has a precise way of being released, otherwise it will be susceptible to being broken. In other words, unless you can surmise the way the hinges come off safely, and with what tools, your best shot is to take it in to Best Buy and see if a tech would assess it for you, and just explain a safe procedure- they're usually very helpful, since they want to keep or bring you in as a customer. When you do get the bed apart, spray out the inside with compressed air. I would use computer screen wipes (available at WalMart), then a small amount of distilled water on a lint free cloth, to touch it up. Be careful about sprays, if you use them, and try to use them sparingly. Camera lens cleaners are good, but it takes quite a bit to cover the glass.
I, personally wouldn't recommend this for someone who hasn't done it before, unless they're very cautious and mechanically minded. However, with some patience and caution, it can be safely done. Wish I had more details, but not having this brand, and not knowing anyone who does, this is all I can provide. Good luck, and be careful and meticulous, if you elect to do it yourself. As an afterthought, you might call Best Buy and see what they would charge, or if they perform this service. I used to work as a tech there, and they're usually more reasonable than dealers, from what I've seen.