Ok. All you have to do is get the first battery out, which is welded on the top prong. If you cant pry it off, just find some way of rigging it up so one side of the battery touches one prong, and the other side touches the other prong. you could even take wires, solder them to the prongs, then solder them to the battery, and attach the battery to the side of the cartridge with some hot glue or double sided tape.
I had to fix the battery for my FF2 cart (soon to be sold on ebay, once I get my SWCDX2), and I just used a twist-tie to keep the prongs on the battery.
Anyways, good luck.