I think you will find it self-explanatory once you take off the front piece. The glass is a small circle that fits on top of a round grommet/washer. There's a recess for the glass on the inside of the cover. Remove the pieces of the old glass, place the new glass on top of the grommet, make sure the grommet/glass is centered over the sensor (or place it inverted into the recess and the cover) and screw the pieces back together. The glass is not glued or anything - it's held in place by the grommet and the recess. Very easy - just don't get fingerprints on the new glass. (NOTE: Photo is of ASI 290 but it uses the same replacement glass)
Good luck.