As I do not have a manual (and you do) all I can suggest is you read it through several times slowly before you try.
The other thing you can do is link out the circuit(s) in question at the panel then remove the cables. But that said there is the slight possibility that whoever installed it used the same cable for more than one device, i.e landing pir and landing smoke, if this is the case the landing pir will not work if you cut the supply off to the cable. (This is because it supplies both devices) You could even buy new 12v smoke detectors and change them. Whatever you do. Mark the cables, and take clear in focus pictures before you do anything.
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