As far as cleaning the ''inside'' of the glass, such an action shouldn't be necessary as nothing actually touches it. However, if the inside is smudged/dirty, you may want to make a repair request to Epson and see if the warrenty will cover it. Such an occurrance is rare, and is usually attributed to manufacturer defect.
Hope this helps,
P.S. If you still want to open it up, you can unplug it and remove the screw on the back of the unit. After that, you have to carefully pry the bottom off (it's ''snapped'' on) and reach up in. However, doing so will most likely damage the scanner, possibly irreparably.