To replace the water pump, just follow these steps.
Step 1: Make sure the motor is shifted to NEUTRAL
Step 2: Tilt the motor all the way up (it will be easier to remove the lower unit like this and will prevent it from just falling on the ground)
Step 3: Remove the trim tab (mark it's position so you can install it in the same spot)
Step 4: Remove the bolt that is located inside the trim tab cavity
Step 5: Loosen the mounting nuts (or bolts) that mount the lower unit to the midsection. Do not remove the nuts completely, but loosen them all the way.
The lower unit is now free to come off. When the lower unit is loose and falls down to the loosened mounting nuts, you can take the nuts off.
Drop the lower unit just enough to remove the small plastic tube from the front of the lower unit. This tube supplies water to your speedometer.
Step 6: With the lower unit removed, you can see the water pump housing. Remove the hardware holding the housing on and lift the housing.. Under the housing, you'll find the impeller.
Remember when installing a new impeller to twist the impeller into the pump housing and ensure that the fins ‘curve' in the same direction as the old impeller.
Remember to re-install the square key back into the drive shaft and impeller if it has fallen out.
Assemble the pump and lower unit in the reverse order of disassembly.
That's it. You've just successfully replaced you impeller.