On my 48 coupe, there is a little clip under the arm that covers part of the drum. You pull that little clip towards the end of the arm (away from the drum) and lift the arm off the drum. Sometimes you have to pry the arms up a bit to get them to move, just be sure the drum lock clip clears the drum.
My 50 truck has a nut on the top of the arm. You use a 3/8" wrench to loosen the nut and the arm usually lifts off.
I presume you have one of these 2 attaching methods, otherwise I guess I need to see a picture of your arms. Gene