If its similar to the 125, then there are 4 or 5 very small cross head
screws which go upwards into the top cover. Without taking these out you will only damage it. Once the top is of, I think the circuit board then needs to come out. I seem to remember ending up with a dish of 7 or 8 screws by the time I reached the batteries.
Might be worth a phone
call to Raymarine
before you start to see if there is a battery
inside the 120 series.
As for the screen
, be careful that coating is known to be fragile.