I've changed a bunch of them over the years. Take the prop off as mentioned. It will make it a little lighter to handle. Next shift it into forward gear before you drop the lower unit. This will enable you to turn the prop shaft and align the splines on the drive shaft and the splines in the crankshaft when installing much easier. Remove the trim tab under the cavation plate if you have one otherwise there will just be a flat plate there. There is a bolt under the trim tab that has to be removed. This one gets a lot of people because it's hidden. I like to always get the complete water pump kit that has the wear plates and the impeller housing or you can just replace the impeller. When you install the impeller make sure that it goes back in just like the old one came out. You will see what I'am talking about when you take it out. The vanes on the impeller are a tight fit in the housing and will be folded back from the rotation. Just make sure the new one goes back the same. I like to use a little white grease or vaseline on the new impeller when reinstalling just for the initial startup lubrication. It's really not that hard other than lifting the lower unit. I got an old milk crate that I stand mine in to hold it up while I do the work. I live in Walker Flippin if you need any help just let me know.

Jeff