Disconnect all cables and turnaround the FS1r. Unscrew all 11 screws of the ground plate and the one on the front at the number pad, shown on the picture.

Remove the ground plate and be careful to touch none of the circuitry, especially the ICs and the brown power supply board with its parts which can be give you an electro shock even while being disconnected, you have been warnend and Block 4 is giving no gurantee nor liability.

Its now easy to peel out the battery, marked in the picture with the red dot. Just push it a bit from the open side of the holder to the opposite side, or in the picture to 'south' and it should jump out. Insert a new CR2032 with + facing up.