Remove the plug above the trim tab (the fin just above the prop) and remove the bolt that holds in on, there is a bolt underneath the trim tab that goes in from the bottom. Just forward and on each side is a nut and washer (underneath the antiventilation plate) On the sides at the top are two more nuts and washers, and up at the very front. First step is to unhook the speedometer tube at the splice, then remove the nuts and bolts ( I prefer to leave the shift handle and gearcase in neutral) and pull the gearcase off, straight out of the midsection.
Remove the three nuts and washers and one bolt from the pump housing pull the housing up off the impeller, the impeller may stay in the housing, if it does, watch for the impeller key to fall out so you don't lose it. I recommend that you replace all gaskets and wear plate (the plate under the impeller) if it has grooves in it, and inspect the housing for any signs of overheating or melting. Install the new gaskets and wearplate, then while holding the key on the flat of the driveshaft slide the new impeller down the shaft, line up the key way and push it down over the key. Smear a little light oil inside the metal cup in the housing and install it, slide it down to the impeller and while turning the driveshaft in a clockwise direction(looking down from the top) push the housing down over the impeller. Install all the nuts ,washers and the bolt and tighten to around 50 inch pounds, (tight but not so tight as to distort the housing). There should be a plastic tube the fits into the water pump outlet hole, it is an alignment device to ease installation of the water tube.
Grease the driveshaft and shift shaft splines with marine grease and install the gearcase. Pull the ignition key and kill switch lanyard to prevent starting and have a friend close at hand. Put the driveshaft straight up into the midsection and into the bottom of the crankshaft while lining up the water tube and shift shaft,(don't forget the speedo tube, thread it thru the hole) (be sure drive and control handle are in neutral) If the crankshaft splines do not line up have your friend rotate the flywheel a little clockwise (by hand) until it lines up. You will have to install the nuts and washers on the sides just before the gearcase seats against the midsection. Tighten them up a little and then while turning the prop (by hand) have you friend shift the control handle to be sure it will go in and out of gear, if not drop the case back down a little and line up the shift shaft. Tighten all the bolts and nuts and hook the speedo tube back up and test run.