You will first need to remove the puncture pin, a large flathead screwdrive will work. The remove the small screw located next to the brass screw ( the small black screw holds the valve in place.
This is where having a valve tool comes in handy. Remove the valve and the cup seal will fall out. Replace the cup seal and re-install the valve, but first put a fair amount of oil or greas on the orings. Install the puncture pin and you are done.