Easy, just snap the threaded metal part off the black alloy top. Push it sideways, it is a press fit.
I have previously posted how to 'fix' these caps along with photos of them pulled apart. Try searching my posts.
You will find a black plastic spring loaded 'key' in there that should engage with a slot. The spring wears or breaks and the key no longer holds. I replaced the key with a screw so that the head engages the slot. My cap is no longer lockable but I can always get it off.