It's doable if you have the proper tools.
You need the special security bit to remove the three screws. The case is polycarbonate, and is welded shut. There's a gasket between the the case halves. Break the weld without damaging the gasket. Remove the battery assembly, remove the two batteries by breaking the spot welds on the nickel strips without damaging the nickel strips.
Spot weld the new batteries back into place, replace the srcrews. Put the gasket back, and weld the case shut with and ultrasonic welder and a polycarbonate rod. File down the seam.