You'll probably find the shoes have stuck to the inside face of the drum. This often happens if you leave the handbrake on. To get to the brakes:
1. Chock the trailer and take the handbrake 'off'.
2. Loosen the wheel bolts, just a small amount, before
jacking it off the floor. Probably M12? So either a 17mm or 19mm socket, as the bolt head varies depending on wheel bolt supplier. Jack the trailer up, put an axle stand or similar, under there, just in case your jack gives up on you. Then undo the bolts and take the wheel off.
3. You'll probably have a 'bearing saver' fitted too. This is a chrome, cylindrical, pressurised grease reservoir, fitted in place of a dustcap. You'll need to remove this. It's pretty easy. Use a soft faced mallet (or piece of wood between a normal hammer and the bearing saver) then give it a stern tap, rotate it ninety degrees and clout it again. Repeat until it drops out.
4. You'll then see (probably under some grease) a castellated nut, with a split pin through it. Remove the split pin. Undo the castellated nut (anti clockwise), then remove the thrust washer under that too.
5. Give the brake cable a good wiggle, sternly tap the side of the drum (at 3 o'clock and 9 o'clock), to jar the shoes off the face and then pull the drum off.
6. This will then give you access to tha brake shoes to clean them and copper slip them as necessary.
Any probs, feel free to drop me a PM.
PS. As Festinghouse says - you don't actually have to remove the wheel, but sometimes it makes life easier!