it was a bit more trickier that I expected but the procedure was a success after all.
Here we go, we need first a F-91W
four screws for a size 000 Philips screwdriver
remove the screws and the case back
hmm, that looks strange
ok, I have seen it before, the paper is to avoid any metallic part other than the two levers for the alarm, otherwise the would shortcut and the alarm would not work at all, so never remove the paper, and even if you do replace it with an equal size one self adhesive.
but how to remove the battery from in there?
it looks impossible, can it be? a casio that a battery can not be replaced? no, I dont think so
so lets remove the whole module from in there
sweet little face
aha, what are those four levers around the module? four levers holding the whole battery cover from the sides on the module, simple and stable I would say
so lets start poping up those levers with a flat screwdriver
one up three more to go
2 up and 2 more left
and the battery cover is finally released
you can see clearly see the 4 levers holding the cover in that one
just notice that the battery is stuck by a tiny little double sided black tape on the top side of the module, a little force and its lose free.
put a new battery and place the cover on the module, click all four levers to secure the cover on the module
to put the module back in the case you should remove the rubber seal first
make sure that all the buttons are pushed out before attempting to force the module in, as you see the button on the right is in and would not be possible to push in the module
while putting back the module push with a flat screwdriver the pushbutton levers of the module inwards so they will let the module to slide in position
put back the seal and screw the caseback and you are ready