I have a Yanmar 2GM20F which also has the aft facing water pump (what genious thought of that design....?).

Pull the two bolts holding the pump and you can then either rotate the pump and take off the 6 screws holding the cover on to expose and remove the old impeller, or do what I do: disconnect the input/output hoses and take the entire pump off as its much easier to deal with that way.

Remove the cover and gasket carefully so as not to ding up the mating surfaces (which could cause a water leak). I use two screwdrivers, one on each side of the impeller to carefully pry it up and out. Ive found that putting a cable tie around the new impeller and tightening it up to compress the blades makes getting the new one in easier. The tie will slide off as you slide the new impeller in.

Put a few drops of liquid detergent around the edge of the pump and lay on a new paper gasket. I also squirt some into the pump to ease the blades on start up without water in the pump.

Replace the cover and screws and reinstall the pump. I have found it makes no difference which way you position the blades of the new impeller as they will sort themselves out without any damage as soon as the engine starts.

See how Don Casey does it in the related link. http://www.boatus.com/boattech/casey/09.htm