your best bet(after you unplug the electrical)is to take the trap apart,unhook the dishwasher hose(if you have a dishwasher)then you can turn that ring with the sevice wrench or screwdriver to drop the disposal.always put your knee under to support it when you are doing this and putting it back.there are three screws that you loosen and a retaining clip if i remember correctly.
put plumbers putty where the strainer meets the sink.