How do I disconnect and reconnect this?
That is a gland connector which seals the outer cover of the cable and grips the protective outer wires then the conductors continue into the box / device to the connections.
So you have to undo the internal conductors and then release that cinnector.
Make sure the supply is off.
We always used to undo the internal fixing nut and the connecting wires and leave the gland assembled to the cable - much easier.