1. unscrew both philips screws inside the battery compartment (the ones that are below the batteries). There are two more under the front glide-pads, these go off easily with a sharp knife. Now open up.
2. in the top cover, unscrew both remaining screws. Then release the blue part of the cover a bit by pressing the two latches in the middle. Now shift the whole blue cover forward (button pins must go past the plastic holding them back.
3. Look at the black plastic top cover. You will see the metal plate of the battery terminals. The part below the left button will be lifted a bit (or may already have moved down by all the handling so far. Push it flush with the plastic top, and put some superglue between metal and plastic to lock it in place.
4. reassemble. Done