How do i fix the thermometer in my vauxhall corsa

He said thermometer....... not thermostat ! the thermometers on corsas always fail with a bit of water, they're in your lower front grill, drivers side.

That is not your thermostat, that has to be the temp sensor on your cylinder head, should be next to the intake manifold on the head. it has 1 cable. that sensor´s only job is to show you the temp of the engine on the temp dial in your instrument display
the thermostat is connected to a water hose that goes to your radiator, its also on the cyinder head, but thermostats are only mechanical doors regulated by temperature that allow hot water from the engine go cool down on the radiator.
let me know