Disassembling PIR Unit for Black Fresnel Lens Replacement

[dotyu]

The place we are setting up is black colored theme and I am using the PIR Unit. Painting the PIR Unit black is not an issue, but it got the White Fresnel Lens (the white dome thing). So I would like to replace it with a black one.

I have been trying to open up the PIR Unit, but I have a bit of difficulty disassembling it. I am afraid that if I apply too much force, it will break. Or is it ok to apply more force until the whole thing is split in half? Is there anything holding inside? Must it be open in a special way?