it should pry open. there is a chance they seal them up, because it is a safety conern for them, but one way or another there is a way to get it apart. take out all screws (some might be covered by stickers) and jamb a flatblade screwdriver between the cracks, and pry. you likely will put some marks in it, but it should be no bother since the battery is junk anyway. just be careful, you dont want to jab or stab the cells inside, they are durable but you know how things go sometimes... for some encouragement, ive never had a battery blow up on me and ive done some pretty scary stuff to them...
once you get in there you will see the batteries are attatched to a little PCB with an I/C chip on it. resuse this! you MUST have it on there, or your battery pack WILL explode when left on charge. that little chip controls when the cells hit maximum voltage and cuts off the charging.
other then those keys, its easy. also, best to get replacement cells with tabs already on them, that way you dont have to risk overheating the cells when you solder them together.