It depends on what type of hub their trailers come with. There are several different types. If you have the type with the rubber plug in the center of the metal hub dust cover (TieDown super lube) what you do is jack up the wheel on that side. Then pry out the rubber plug and carefully (because the inner edge of the metal dust cover can be sharp) clean out the old grease to reveal the grease fitting in the spindle. Then hook up your grease gun and pump in grease while turning the wheel, till you see the new grease pushing out the old grease. Then take a clean rag and wipe out the opening in the hub dust cover (this allows some room for the grease to expand when hot) and replace the rubber plug. If the old grease came out looking like the new grease (ie not milky looking) you are done. If it comes out looking like water is mixed in then you have a much bigger job. You then need to take it all apart and clean and inspect the bearings. If you see any pitting at all on either the races or the bearings I'd change all of them.
Take a look here:
www.championtrailers.comlook in the tech articles.
I don't know for sure what hubs F/W puts on their factory trailers but the Tie Down Super Lube system is about the easiest one there is to maintain. I've had an axle with these hubs for 10 years and its just the best, easiest way to grease trailer bearings.