Back moved aside

View after putting rear part aside. 2 screws for the board and sticker for battery.

 

1. Undo two screws.

2. Remove the transparent lens "cap" from front (its glued to front)

Use something soft (cotton swab, pencil eraser) and push the lens inside - it should smoothly come out together with the PCB.

3. Optionally peel off the battery.

Done