Hello, Welcome to Just Answers! I will assist you with your question. The key is actually sealed very well. But with some patience and a steady hand can be opened. The unit will open where you see the split where the 2 pieces fit together. you can try to pry these pieces apart but I find using a sharp utility knife to cleanly cut around this split and through the seal works best then once you have cut around the outside you can use a pry tool or small flat blade screwdriver to pry the pieces apart. The pieces are glued together so you will need to go slow and easy to get them to separate. once opened replace the battery and using some super glue reseal the unit.
The battery is also soldered into place so you will need to unsolder and solder in the new battery.
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Thank you - this is helpful. One more question; after I finally got it open (and it looks like I ruined it as the battery came out braking this 'thing' that is attached to the battery) I'm wondering what the battery is. Another site has given me CR 2025 but it looks too small...
I believe the last one I did used a Panasonic VL2330-1HF.