Get an 8mm allen wrench & remove the clutch cover bolts. Depending on year model, the cover will either be sealed with silicone or an o-ring gasket. If it's siliconed, you will have to pry it off, if it's the o-ring, hold ya hand on it so it don't fall off. When ya remove the cover, look inside to see if grease has been slung everywhere. If there's grease, then the seal is blown out.
To replace it, remove the rest of the bolts & the lower unit. Stand it on the prop end, being carful not to hurt the prop or threads. Remove the 2 allen head set screws that hold the gear & cone together. Use a rubber mallet to knock the gear loose from the cone by driving the gear toward the housing. When the 2 seperate, remove the cone, then the gear. The seal will be exposed for you to change.
Deadbird8 has a very detailed post about it on the MB forum if you need step by step info.
FYI, the seals from MB are not very expensive, but definitely worth the $.