Edgar - Pretty easy, although it is a V-Drive so the impeller is facing the Stern of the boat. I use a mirror so I can see what I am doing. I lie across the intake and remove the flame arrestor to make it a little easier to get to. Remove the 4 screws, spray some WD-40 around the impeller. Bump the starter a few times to lube the impeller making sure not to start the engine. I use a paint can opener and work the impeller from each side to slide it out. Slide on the new one, remember it is a splined shaft, replace the gasket and re-install the screws, done, pretty simple job, about 10 minutes and only one skinned knuckle!
Ballast impellers are simple, you should have 3, two are behind the access panel in the stern rear lockers and the KGB should be near your transmission. Green is best!