How do you replace a key on a casio 100 keyboard. which screws do you remove in the back

Question moved from Cars and Trucks to a Casio Keyboard one.
Easy enough if you can find the part.

https://www.google.com/search?q=casio+wk-1250+parts+diagram

casio WK1250 Parts and Accessories - PacParts

https://www.pacparts.com > model > model_id=WK1250
Mfg / Model No, Description, PDF, Year. casio WK1250, Musical Instrument: Electronic Keyboard, 1998. The following parts are available for the model listed ...

Casio WK-1250 Repair Parts and Accessories - Syntaur

https://syntaur.com > keyboard > keyboard=2080
Casio WK-1250. Released in 1998. The WK-1250 is a 73-note consumer keyboard with built-in speakers. Synth. All photos are copyrighted by Syntaur, ...

Casio Keyboard Junkyard: Welcome

https://casiokeyboardjunkyard.com Used accessories, parts and stuff for Casio Keyboards ... CASIO WK-1200, WK-1250, WK-1300, WK-1350 models are basically the same and use the same parts.

Casio Repair Parts - Keyboard Kountry

https://www.keyboardkountry.com > keyboard-parts
Casio Repair Parts ; Yamaha Brand Key Lube (Small). $3.80 ; Casio CTK-560L Ma1M Light Boards. $9.95 ; Replacement Keys for Casio CTK-491 and Others. $10.95.
Missing: 1250 ‎' Must include: 1250 ..

You need first of all to remove the cover, access the keybed, remove from keyboard base and then you will be able to remove a key - it is impossible without following this procedure.

What model? Diodes are either burned up (but you would see the ends still present) or missing because they aren't applicable to that model. It's more likely the rubber pads under the keys need to be replaced

These pads can be replaced ( providing Casio still have spare parts ) There are a lot of screws to undo to get inside the keyboard, then remove any PCB above the pad, replace and reassemble.

or

Contact your nearest Casio service centre

you nearest casio dealer may have one in the back of the store,
many buyers don't want/need the manuals so some stores hold on to them for such occassions

Read your manual regarding "fingered chords" and similar subjects. It sounds like you are in a single fingered chord mode...

The contacts are under the keys, about halfway from the front of the white keys to the back of the key., There are several different key types for Casio keyboards. USUALLY the keys are either in blocks or individual. Here is a manual for a Casio, and while it is NOT your model, your keyboard will be very close.

http://elektrotanya.com/casio_ctk-220l.pdf/download.html

There is an exploded voew on page 16 of the key assembly.

I just fixed my CDP 100 which had a similar problem - dropped and two keys not working up one end.
1. Lay keyboard upside down on soft surface. Under keyboard is plastic cover held on by 17 screws. Remove all these and lift cover away. The metal key ends are all revealed.
2. Locate the disloged keys. Carefully press them back into position - they will "click" into place. Make sure the corresponding key on the keyboard isn't stuck in an "up" position - you may have to remove the metal key part and replace it again while making sure the key is correctly seated.
3. Replace cover and screws.

It may due to faulty circuit board of the keyboard. Check for warranty and ask the vendor to replace the keyboard.