Agree with rgano. This is an exaggeration but only sort of. Basically when a traditional stuffing box develops a leak, it's a nuisance thing and you re-pack it (in place). When a dripless seal fails the boat sinks. That's extreme (and obviously doesn't always happen), but I say that because there is no way to "re-pack" the bellows. The leak can only get worse, and it won't necessarily be only a nuisance drip.
To replace the bellows, you need to pull the shaft and coupler apart, and then either slide the shaft back or remove it completely (depends on your space). If your cutless bearing is old, you might consider just pulling the shaft and replacing that too (again, depends on your boat's configuration; if you have a strut you may be able to do the bearing with the shaft in place).
Dripless seals are relatively maintenance free, but they do still have a maintenance schedule (e.g. replace bellows every X years). I don't know what kind you have, so can't speak to specifics.
If you hire someone to do it, you may want to check on how they plan to accomplish it. I have read about (for example) people using hammers to get the coupling apart and thus damaging the transmission case. So try to get a feel for how they plan out the work.
Then when you have a new seal, check out the maintenance schedule posted by the mfgr (for future reference).
(Yes, I've been boat shopping and have seen quite a few boats with the original, 20+ year old dripless seal because "they're maintenance free!")
