Earlier this year i found out about this small watch. It's only 10$, which is more than most of us spend on coffee or movie ticket so i'm pretty sure anybody can afford it. Display is easy to read (really clear, better than some more expensive models) but as long as there is light. As soon as it gets dark and you want to see what time it is, you will get disappointed. With its dim green LED, you will barely see minutes and seconds. So, instead of searching for an alternative, i did this simple mod to make it more readable (and for those who want alternative, check out Casio A168).
This mod won't affect water resistance (F91W is only WR - Water Resist but i've seen some people swimming and diving with it) and it may barely impact battery life (considering i didn't change its 100 ohm resistor). I am using modded watch for about 5 months now and i didn't have any single problem, i even take showers with it.

Since my main language isn't english and i still have to improve few things (and take more photos next time i disassemble watch) i recommend reading this first. It shows disassembly with a lot more details. I will also try to include many videos and guides for those who don't have experience with electronics/soldering.

Supplies

- Casio F91W (Should work on other models with "Illuminator" type backlight, but others might already have 2 LEDs, like AE1200 for example)

-Tweezers

-Screwdriver

-Soldering iron, solder and flux

-Small gauge insulated wire (you can find it on el. motors, speakers etc.) - non in copper wire won't work, you will short out and damage your watch

-0603 or 0805 SMD LED (something close to it, if it's too big it won't fit)

- Glue (optional)

- Electrical tape

Step 1: Disassembly Time

This should be easy for everyone, you don't even have to remove strap.
Just flip your watch on face (put some soft paper or tissue under it to prevent scratches) and remove 4 screws that hold backplate.

Once you did that, remove backplate and small paper that is on back of the module. You can also remove rubber seals (and replace them later if your watch is old).
Pull the module out carefully.

This video should help you if my instructions aren't enough as it shows how to replace original LED (this modification is similar, but i just added another LED in parallel)

Step 2: Module & Soldering New LEDs

Once you got module out, take tweezer or small flat head screwdriver and pry off metal battery holder. Battery should fall out by itself, remember polarity so you can put it back after (negative (-) is facing PCB, positive (+) should be facing metal holder).

Once you did everything, you can pull PCB (Printed Circuit Board) and flip it around.
You should see small led on one side of the board (i got picture from bobyong808, i hope that's ok, if not, message me so i can remove it). Desolder it, we won't be needing this anymore (you can damage it, just be careful to not damage PCB). Hot air station would be good, but this is easy to desolder with standard soldering iron. Helping hands could be also useful since SMD LEDs are really small, maybe even get a magnifying glass.

I recommend watching video from GreatScott about soldering SMD components, this will help you if you don't have experience with SMD soldering.

Once you removed it, solder your new LED in place (positive of LED should go to contacts/button, negative to resistor). Fine solder and flux really helps when soldering SMD components. Use the picture as reference to where LEDs positive side should be. Get another LED and solder small gauge insulated wire to it, this will be tricky so take your time. Remove insulation from wire first by scratching or sanding few milimeters of the ends. I found this video about this, it may help you with it. When you are done, solder it parallel to the first LED (connect both positives together, connect both negatives together). Put small piece of electrical tape under LED so it won't short out anything, you can also glue LED in place, but this is optional as i didn't glue mine and it doesn't move around.

I got my LEDs from Nokia 5110 display so if you have huge pile of old electronics, you can search for older phones or electronics that has SMD LEDs, but if not, they are really cheap (you can find 100pcs for dollar or two on ebay).

Step 3: Assembly

Now put everything back together, second LED should fit right under white plastic, there should be space on right side wide enough to fit LED. Put battery back in (negative (-) is facing PCB, positive (+) should be facing metal holder). Once the metal holder clicks in place, your watch should work (and start from 12:00:00), if not, check if you placed battery correctly and if you didn't forget rubber contacts on display. If they are dirty they might be getting bad connection and display glitches or display nothing at all. Test to see if backlight is working, if it doesn't you might have flipped LEDs polarity or you might have burned them accidentally.

Be careful to not invert backplate since small piezo electric speaker is on back of it which produces sound when you click and alarm. There should be small piece of metal that is in contact with backplate, align it with speaker.
I also put my F91W on nato strap, it's amazing how light this watch is. So comfortable that you will forget that you are wearing it.

This is it, if you did it, congratulations.