The seal there usually leaks because the shifter has taken a hit and damaged the seal. This sometimes even leaves the shaft bent, so that's worth checking too.
The seal can be pulled out without any need to split the cases. The easiest way is to do what everiman said, take off the covers, clean all around the shaft and seal. Lean the bike over on something large and soft (passing children work well...!) so that the oil is on the other side of the gearbox.
Find some self-tapping screws (or some wood screws, it doesn't really matter) that are not bigger that the width of the seal.
Using a drill, about half the size of the screws you have, drill three holes at even spaces around the seal, closer to the outside edge than the centre. Screw in three screws, and use a set of locking pliers to ease the old seal out.
Don't throw away the old seal. Use it as a drift to protect and ease the new seal in. A short section of pipe, that is the right size to put pressure on the outside rim of the seal is the right way to push the seal back in. If you use a flat ended punch or something like that be very careful to take it slow and tap around and around the seal. Before you do, make sure there are no rough edges on the end of the gear shaft and put oil on seal and shaft to make sure it slips on without damage to the seal lips.
If the shaft is bent I have been known to ummmm persuade it back straight by putting a pipe sleeve over and using a big hammer to bend it back. CAUTION!!!!! Do this at your own risk!!! I have got away with it several times, but there is a big risk of major damage to the cases and shifting mechanism.
Nigel in NZ
The mouth of a perfectly contented man is filled with .
-- 2200 BC Egyptian inscription