As I said this job is very, very easy. If you can change oil or an air filter or clean a blender after making frozen "chick drinks" you can change an impeller.
Step 1: loosen the screws and remove them. If they are frozen or rusted use some PB Blaster but be VERY, VERY careful not to just spray it on! PB Blaster will eat pump seals, engine seals and engine gaskets for breakfast. If you need to use a penetrating oil I'd advise spraying it into the spray can cap and then dab it on the screws with a q-tip.
Tip: Buy yourself some spare face plate screws as one will always drop into the bilge, to never be seen again, and it will do it at the least opportune time. Murphy is always waiting for you...!