This shaver was pretty clearly not designed for battery replacement. To open, first remove the screen and blades, and unhook the metal collar from the tab on the back of the case. Then carefully pry off the black inset on the front of the shaver. Underneath it, near the bottom are two screws. Remove these. The case is now only held together by several plastic latches, which can be carefully pried apart with a small screwdriver. The two 2/3A size batteries are soldered to the printed circuit board, which can be pulled out (don't break the wires to the motor) by a little wiggling. The original batteries in mine are 1100mAh, and I have just ordered 1600mAh replacements. The new ones don't have tabs - so soldering them in place is probably going to be a challenge.

Good luck.