How to Change water pump impeller 40hp mercury?

First find your year model and aquire a impeller kit as it will have all the replacment parts needed. To change the impeller you need to remove the lower unit. Put the motor into forward gear and remove the bolts/nuts holding the lower unit to midsection. Note that there is usually one bolt under the trim tab, usually it a total of 6 mounting bolts/nuts or less depending on year model. Once bolts/nuts are removed, the lower unit will drop down. The pump housing will be right on the top of unit with drive shaft protruding thru it. The pump housing is removed, then the impeller can be removed from the shaft. Retain the flat key and reinstall it with the new impeller,gaskets and wear plate. Lube the housing with some petroluem jelly or white grease, while pushing housing down turn driveshaft clockwise to seat the impeller in housing then torque nuts to 100 inch pounds. Reassembly is the reverse but make sure you line up the shift splines.