At least, they're easy to replace if it's a Shimano hub (and possibly in the case of a Campagnolo as well). Various other hubs require special tools to remove the freehub body, and some can't be serviced at all.

If the hub is a shimano, and light oil (even ProLink or something like that) doesn't clear up stuff inside the freehub mechanism, and you need to replace the unit, a 10mm allen wrench loosens the bolt that holds the freehub to the main hub body. You'll need to remove the hub's axle in order to get in there.