You can't fix it till you know what's wrong with it.
There is no way to change gear ratios.
Replace the instrument cluster.
It is not repairable, you should replace the instrument cluster.
depends on whats wrong with it
These parts are only fixed by replacement.
remove the battery terminals for a minimum of 10 minutes.
There is no fixing a broken motor mount. Replace them with a new ones.
you have to keep your pen near something hot....
You would fix it by replacing the instrument cluster. The speedometer itself may not be faulty, it is more likely to be a problem with the speed sensor.
You would need to be more specific with your question. You are not providing enough information to supply a meaningful answer.
Check fuses If you are getting power to socket, but not through it ,replace socket