I replaced a defective one in my Superformance some years back, and here is what I did.
Removing the drivers seat is optional, I didn't and don't think it necessary. Remove the steering wheel and then remove the speedometer. With the speedo out you can reach in behind the dash to remove the water temp gauge from the dash. Then remove the sensor from the intake manifold and remove the gauge and attached tubing and sensor from the vehicle. You kind of have to work with it to get it out. To install, just reverse the process, being very careful not to get any crimps/kinks in the tubing.
You might want to test the new gauge out before installing. I did by boiling water on the stove and putting the sensor bulb into it. Better to know before you go through all the work to re-install, just saying.
If you haven't removed any of the gauges yet, they are held in place with two little knurled nobs that secure a bracket against the rear of the dash. Stick your hands up under the dash to the speedo and feel for those little knobs, or use a mirror to look up there to see them.
