I saved my original 12V socket, so I checked it out.
Gigitt is correct it is keyed so you cannot turn the top until you loosen the nut on bottom.
The nut is a slotted round nut (plastic).
If you cant' get it with some small channel lock pliers, try a flat blade screw driver. Put the blade in one of the slots and push on it to turn the nut.
If you are replacing it with a usb charger/voltmeter like a lot of people have done, it will probably be slightly larger than the hole.
It could be forced in but probably better to enlarge the hole
slightly.
I just used a round chainsaw file and lightly went around the hole, it really doesn't take much.