Check out this past post for some good generic info on impeller replacment.
http://forums.iboats.com/showthread....+replace ment
Check out the link within the above thread and there's a real good rundown of how to replace an impeller on a larger motor. The general stuff still applies although it'll be a tad simpler on your little 5HP.
Use this forum's search feature and you'll find lots more.
Note impellers are generally not that expensive, can't give you specifics without a serial #. I'd recommend looking up your motor at the link I previously posted and you'll not only get good parts info but also illustrations showing how your motor's lower unit hooks up and whether or not you'll have to deal with a shift shaft.
HTH...........ed