This is a brief guide about opening the scanner body and cleaning the internal side of the glass. After some years, the scanner works perfectly, but some tiny particles can accumulate inside and produce strange little spots over the scanned images.
You will need:
a Phillips screwdriver (cross head, to remove screws)
a little flathead screwdriver (to separate parts)
a clean microfiber cloth
a glass detergent or water for extra safety
Open the cover.
Remove the 2 screws.
NOTE: there is no need to unlink the flat cable that connects the top with the main body, but be careful to do not stretch it.
Remove the black plastic front cover by first releasing the small clip at the bottom and then pulling it up. It may be easier to flip over the scanner and use a flathead screwdriver as lever between the gray edge and the black front cover.
Remove the 2 other screws.
Then, push the plastic frame containing the glass towards the other side (where the front cover was mounted). You can insert the flathead screwdriver between the grey frame and the black glass frame and use it as a lever. Do it gently to avoid damaging the plastic parts.
Now the glass frame should be released and you can lift it up and clean it.