Remove the nut and washer from the top of the cup.
Remove the cup. Few come off easily. The last one I did came off easily after I power washed around it. I imagined I'd broken any suction and cleared any build up of grit. In the past we have gripped the lip with a pair of long nose pliers and ‘unscrewed’ them. The easiest way is to drill out the ‘core’ of the cup with a 22mm hole saw. This will expose the second nut below the cup.
Working both sides of the boat, use a large screwdriver and long reach socket to remove the nut from inside the boat and withdraw the bolt from the outside.
Remove the deep black rubber sealing washer from the inside of the boat.
Clean up the whole area.
To replace the cup:The procedure is simply the reverse of the above but do note:
The order is – screwdriver headed bolt / brass insert / black rubber seal / washer / nut / cup / washer / nut.
A small amount of Sikaflex or similar polyurethane sealant on the rubber sealing washer is useful but should not be necessary.
It is necessary to tighten the first nut fairly well but you do not have to really tighten the one on top of the cup.