dead easy,
take prop off, undo big nut on pinion, take out drive flange, prize out seal, knock in new one. put back together.
potential problems,
siezed prop bolts, soak with WD40. big nut on flange very tight, brace with pry bar using nuts and bolts in two of the holes in the drive flange, and use a very big breaker bar on the nut. also the drive flange may need replacing depending on how bad the seal has been rubbing on it.