From a Honda Mechanic:
Under the hood at the crankcase end (front) you will see hopefully a square box with wires (voltage regulator for charge coil if equipped) and to the right of this there should be a rod and arm attached to your gear change lever, put it in forward gear and it should raise the arm - you need to remove the pin through this to disengage the gear change rod. (pin is secured by a split pin in the end)
Once done undo the five bolts under the anti cavitation plate that hold the gear unit to the leg. Drop the gear unit off, and on top you will see the impeller housing with the driveshaft sticking out (or more likely the drive shaft will be stuck to the engine and still in the leg). Undo 4x 10mm head bolts, pull the impeller cover off and remove the impeller from the pinion shaft - don't pull the shaft out or you'll be needing to dismantle the gearbox to put it back in! There is a metal plate under the impeller with gaskets either side. it;s worth replacing these and seeing how blokced up it is beneath the plate at this stage.
Putting back together is the reverse, but as your putting the impeller cover back on turn the shaft (with the impeller on) clockwise and lower the hosuing onto it, cover it in grease to help as well.
Hope that helps - it's a damned fiddly process so it may be worth getting an expert to help, should only take an hour max. (much less if all is well)
Alan