The Honda manual would recommend draining the oil, pulling the R. cover off, then the clutch, then the splash guard. Pull out the shaft and then change the seal.
Sounds like a long boring job.
If you're cunning you can change the seal without pulling the motor apart, or even draining the oil.
Lean the bike right over on it's side so that the oil drains to the right side of the gearbox. Get a couple of those handy picks (little pointy things), and you can dig the seal out and just pull it off the shaft. Be super careful not to scratch the seal housing. Then just pop a new seal in and tap it into place.
Recently I wanted to skim 0.5mm off the little steel bush that lives in the primary driven gear assy on the SP clutch. Normally you'd have to pull off the whole clutch assy, then the side cover etc. Lots of messing around. Using the above method (or a similar one) I just whipped off the clutch, dug the seal out, sorted the bush, and it was all back together and running in half an hour.
Cheating I know, but a couple of quid for a seal is less than I'd have charged for pulling the side cover off, new gasket and what have you.
Go the long route if you want to know the inside working of the mission, but if jobs like this bore you then cheat.