You could pull the blue plastic plug out and pour bleach down there. Problem is, even if you flush it down with 20 gallons of fresh water (which I highly recommend) you might still leave some bleach on the casing, droppipe and wire. I probably don't have to tell you what bleach does to metal.
If you want to do it right, you would loosen the four nuts. No more than two turns. Then finagle the well seal and droppipe up far enough to pour bleach and water (in that order) down the pipe being sure to rinse all the casing, pipe and wire.
This is going to be very heavy.