Open the front cover

A photo of the back would be good with the battery removed. Presumably there is a hole in the back for a battery? There maybe a hidden screw in the battery compartment.

Else the casing is clipped together with hidden clips probably in the middle of the sides? In which case use a spudger pry tool to ease the back off, maybe starting in the middle of the side with the widest gap. A blunt butter knife might work but might scratch the plastic more. Some clips might break.

If it glued even more might break. Possibly slightly warm it first.

Do you want to take the cover off to remove condensation following use in a bathroom. Maybe just put in a warm dry room for a few days. Or drill a few holes in the back?