Trailer drum brakes are not the same as rear drum brakes on modern vehicles. They are like drum brakes on the front of vehicles of 40-50 years ago. Meaning that they are not self adjusting, so you may have to loosen the adjustment to get the drum to come off and you have to remove the cotter pin or locking plate to loosen the spindle nut. They do not just slide off the wheel studs like on a truck rear axle, the brake drum and hub are all one piece.
First you loosen the adjustment by turning a star wheel you can get to from a slot in the backing plate. Then you remove the dust cover and you will see the spindle nut and cotter pin. After removing the pin or other locking device you should be able to loosen the spindle nut. Remove the spindle nut. The nut will have a washer under it don't lose that. Also, when you start to pull the drum off the outer bearing can fall out of the race so be ready for that. When you put it all back together, I recommend installing new wheel cylinders because that is what usually fails with drum brakes. The rest of the hardware and shoes may be fine. However if the brakes are in bad shape, its easier, cheaper and faster to just install a new complete galvanized backing plate with the newer style aluminum wheel cylinders. Also always use a new cotter pin or locking plate. I would also take a good look at the bearings and races, if they are pitted at all, or rusted, then you have to replace both the bearings and races. And install a new grease seal.
Take a look at Champion Trailers look in the tech section.
Also etrailer has a series of videos on trailer repair that are very good.