You'll either need the special tool for unscrewing the threaded gearcase cover, or very very carefully drill the cover in a few places then break it out. If you destructively remove it, take care not to damage the bearing carrier. If this motor is a "Salty Dog" it's very likely the gearcase cover isn't gonna unscrew anyway, it'll be all bound up.
Then, with a slide hammer attached to the end of the propshaft, pull the entire proshaft/reverse gear/bearing carrier assy out of the lower unit. If it's reuctant to come out, apply a bit of heat to the outside of the gearcase and this'll expand enough to git 'er out.
The propshaft will just slide out the back of the bearing carrier assy, leaving reverse gear/bearing assy pressed into the bearing carrier. You should be able to tap the seal out from the back side without removing reverse gear/bearing. If you can't, gently heat the O.D. of the bearing carrier and reverse gear/bearing should fall right out.
Use care when handling the alum bearing carrier as it's quite easy to break or damage the part that the gearcase cover screws down against.
Drive out the old seal and drive/press in the new one. Take care not to damage the propshaft roller bearing while you're driving out the old seal. A screwdriver, bent to a slight angle at the very end, makes a cheap, effective seal-driving tool. You may be able to pry it out of the front but be real careful as the bearing carrier might not be able to take the prying without breakage.
Clean the inner I.D. of the gearcase including the threads for the gearcase cover; clean the O.D. of the bearing carrier, install a new O-ring on the bearing carrier.
Note if the gearcase internal threads are gunked-up with salt or other hard deposits, you'll have to scrub them out with a metal-bristled 'toothbrush' and/or clean out of the threads with a mechanic's scribe/pick. Otherwise the gearcase cover won't properly thread into the gearcase. Before you reassemble it's a good idea to check and make sure the cover threads down properly. This will ensure the gearcase cover develops proper torque to the bearing carrier instead of binding on the gearcase.
Spread a thin coating of Permatex #3 Aviation-type gasket dressing on the threads of the bearing carrier and also the internal threads in the gearcase. Carefully tap the whole mess back into place.
Note before you reinstall be sure that the shift cam plunger hasn't fallen out of the end of the prop shaft. If it's in there loose or fell out & laying in the gearcase, install (with the pointy end out) with a dab of thick grease to hold it in place.
Spread some Permatex #3 on the threads of the gearcase cover and screw back into place. Note if you don't use the special tool you'll have to tighten with a pin punch or equivalent and this is nowhere near as tight as it should be (60 ft-lb). Although it might work for ya, be sure to re-check after 1st test run.
Here's a link to the special tool, Mercury P/N 91-99222T:
http://store.eastcoastmarineservice.com/product_p/91-99222t.htm
Here's a diagram which should help:
http://www.mercruiserparts.com/Show...inbr=467&bnbr=170&bdesc=GEAR+HOUSING+ASSEMBLY
HTH..............ed