The brakes rub on the middle of the side of the drum, so there's a ridge worn into the drum that you have to get past. There's two methods I know of.
1. "they" say that you can open the little access port at the bottom of the backing plate - make DARN sure your car is safely blocked up before getting under there. - anyway open the port and rotate the adjuster to get the pads away from the drum. I have never been able to make that happen.
2. The geometry of drums is set up so that they lock harder when they start to grab. What that means to you is that you can slightly rotate the drum on the wheel studs, and use the geometry to help you get the drum off. If you rotate the drum in the direction it would turn if the car was moving backwards, the pads will more easily move out of the way. I hope this makes sense. So you pull and twist, repeat as necessary. If you twist the wrong way, the pads will lock harder on the drum.
Then when you have the drum off, take a grinder (safety glasses) and grind off the ridge. I hope this helps.