Shift into neutral, remove the kill switch so the engine can not start.
First remove the trim tab. There is a rubber plug on the top rear of the gearcase, remove this and it will give you access to remove the trim tab. Once removed look up into the cavity and remove the bolt. Now remove the bolts on the side of the gearcase, should be 6.
The lower unit should drop down. There may be a speedo tube in the front of the gearcase, pull this tube off with a pair of needle nose pliers. With the gearcase now loose remove the gearcase from the midsection.
The water pump is the large plastic housing around the drive shaft.
Remove the bolts holding the housing on.
Pull up on the rubber impeller and remove it from the drive shaft, don't lose the small key.
If you are replacing the entire pump just replace the parts one by one.
If you are only replacing the impeller, make sure the gaskets are in good condition and the stainless steel wear plate and cup are in good condition too. Small minor grooves are OK. Major wear, the cup and or plate will need replacing.
Put the new impeller on the drive shaft aligning the key with the impeller hub.
Put a small amount of grease in the cup of the pump housing. While pushing down on the housing to push it over the impeller slowly turn the drive shaft clockwise until the housing is seated. Put a little grease on the bolts and reinstall.
Put a light coat of grease on the sides of the splines on the top of the driveshaft.
Reinstall the gearcase is a reverse of the removal. Make sure the gearcase and control is still in neutral. Make sure the shift shaft aligns properly.
If the gearcase does not go up all the way, slowly turn the flywheel while pushing the gearcase up.
Check the water tube in the top of the water pump housing aligns with the water tube in the midsection.
After the gearcase is in position and bolted up, check the lower unit shifts properly by shifting the control while moving the prop by hand.
Hook up the kill switch, start the engine on flushing cups to ensure the pump is pumping properly.
I have enclosed a parts breakdown to assist you.
This should do it. Let me know if you have any other questions.