Most brake drums will have an adjustment wheel inside the drum, accessible from a slot behind the wheel. You must let the brake shoes in for the drum to loosen. After you do that, it should pull right off.
Reference this prior question for a guide and general diagram on how it works.
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If they are not totally froze up...adjust the brakes DOWN (or in) so they are not so tight. TAP all around the drum, (not hard) with something hard, large screwdriver handle, wood, something to help loosen the built up dust. They should come off if they are not totally "welded" together. Good luck
Make sure your e-brake is not on. That will tighten the grip the shoes have on the drum and make it next to impossible to remove. Assuming that wasn't the problem...
A lot of brake drums have threaded holes on the face near the lugs. These holes are there to put in jacking screws for drum removal. Find one or two screws that fit the threaded holes and use them to push the drum off.
If there's no such holes, well, get a deadblow hammer and start banging. Penetrating oil from your local auto parts store will also help.