The choice is easier than you think:
ESTIMATED TIME: 10-15 min (mostly to disassemble/reassemble the device)
TOOLS: small screwdrivers, heat pen/gun/hairdryer, piece of plastic/rubber, ?fingernail
I'm really kicking myself over this one. My first LCD device was my first pocket calculator in grade school, and I experimented the HECK out of it, disassembling/reassembling it many ways, trying different voltages, polarizers, etc -- but I never dared to mess with the "magic box": the rigid glass-encased Liquid Crystal chamber.
In the ensuing decades, I threw out countless devices, some quite specialized or expensive, because "the magic box" stopped displaying completely or properly. I now know I could've easily saved them.
I'll spare you my voyage of research to this belated discovery and give you the bottom line:
1) small device LCDs today are much more casually assembled than they were in the 70s. They are basically a sandwich of plastic layers with chemicals "printed" on them, bound with adhesives.
2) Almost all LCD display failures are either failures in the ribbon cable/connector or loosening of the sandwich.
Ribbon cable/connector failures are a separate topic (almost as easy, but with more options in construction and hence repair). This method only covers the sandwich.
1) Make sure the device is unplugged/safe. Learn what the many forms of capacitors look like (esp. electrolytics). Almost all consumer devices operate on a safe low voltage DC internally -- battery, wall wart/brick or a discrete internal power supply that you can identify/avoid. It's really easy, but if you can't/don't do this, you're on your own. You've been warned.
2) Carefully disassemble the device to expose the LCD display, which may be mounted on either the case or LCD. keep track of those tiny screws, plastic tabs, alignment pegs, etc. Photograph each step with photographs (cell phones are fine) until you are comfortable doing this. It's a bit of a skill, but an easy and valuable one (e.g. never throw away a device without disassembling it first!) be especially cautious around the LCD and its wires/connectors
3) Expose the back of the LCD (non display side)
4) Gently warm the back of the LCD with your hot air pen (hot air gun, hair dryer, or other source of heat), held at a distance. You may only need to heat it a bit above body temperature or slightly warmer than that. It should never get hot! I like to use a piece of cardboard with an appropriately sized cut-out as a mask to protect the surrounding ribbon cable, hot glue, plastics, etc.
5) Using a piece of firm plastic or rubber (U suppose the back of a fingernail might work), gently and uniformly rub the entire back of the LCD to press the layers together and make the warmed laters of adhesives stick together as they did at the factory.
Optional: I like to test the LCD once it has cooled to make sure it's completely fixed (if not, warm and rub it some more) and that I haven't dislodged any connectors or wires, but this depends on the device. On a battery operated clock or voltmeter, this may be safe/easy, but on devices with an internal power supply, it may be safer not to EVER power up with the case open (unless you know what you're doing). Sometimes you have to disassemble the controls (e.g. silicone membrane buttons with carbon pads that make contact with with copper fingers on the PCB) to access the LCD, so you may not be able to press an "on" button; these are easily shorted with a paper clip, but don't try this unless you understand them well. Sometimes you have to remove essential connectors to disassemble the case, so testing is impossible. Each device is different.
6) Allow to cool and then reassemble.
a) I like to use a strip of expired/unsolicited credit card or "membership card" to rub the back of the LCD. You're supposed to cut them up anyway, but tossing them into the trash as a group is just a slight inconvenience to an identity thief) Those rigid plastic cards/strips are useful for many hobby tasks: mixer and application paddles for small epoxy jobs,
b) A friend says she's read this method somewhere, but they suggested using a pencil eraser to rub. I think she's confounding it with another hacker repair technique, but she's adamant. I can't see why it wouldn't work, but I like to make a few firm/gentle parallel swipes with a medium-firm plastic edge. If you like the eraser method, consider using a typewriter eraser pencil (do they still make those?) they're firmer and won't leave as much debris.
Comments and suggestions eagerly welcomed!