First thing is to loosen the drum where it has become stuck to the hub. On the outward facing edge or periphery , the drum is reinforced. A few smart taps with a hammer should lossen this bond. The next thing is that once the drum starts to move, it may become jammed on the brake shoes. All you can do here is to try and ease it past the shoes, it often helps to try and rotate it at the same time, but the shoe steady pins might become detached as well. Before replacing the drum it is often advised to carefully remove the wear ridge on the outer lip of the drum. This is normally done by the careful use of an angle grinder or you could do it by hand with a coarse file. Greetings from the North Pennines and Hadrian's Wall.