There are two ways to remove the seal as mention by Hookup. One is to remove the big nut and the carrier assembly In addition to needing the special tools, note that the nut may not want to come out and may need a lot of heat and/or a chisel to break it into pieces. Even after the nut is removed, the carrier may be stuck and getting it out can be quite difficult. Another tool, a puller is needed and probably heat. So the bottom line is that it is possible to destroy parts when taking the carrier out.
The second approach is the hack again mentioned by Hookup. There are videos online showing different wats to 'pull' out the seal. A small hole can be drilled into the lip of the seat and then a drywall screw is carefully screwed into the hole. A tool like a vice grip is used to pull the seal out. The task can require multiple holes. You have to be extra careful not to scratch or score the prop shaft when tightening the screw, If the prop shaft gets scratched, it will leak even with a new seal and the prop shaft will have to be replaced ($$$$)
You didn't mention make model and year of the outdrive. Some LUs have two seals installed in opposite directions. One will have the metal lip facing out and is easier to drill but the other has the rubber lip facing out and you have to kind of butcher it to expose the metal lip.