Replacing a coolant sensor is a fairly simple repair, but can be a bit of a PITA. Don't worry, a bad coolant sensor isn't necessarilly indicative of any other cooling system problems.

1. Allow your engine and coolant to fully cool
2. Syphon coolant out of plastic reservoir
3. (Optionally) Detach the overflow hose from the reservoir
4. Pry and lift plastic reservoir and free it from the retaining clip (this is the most difficult step)
5. Turn the reservoir upside down, pull out and remove coolant temperature sensor probe from the bottom of the reservoir (pull hard)
6. Disconnect plastic electical connector and discard bad sensor

Installation is the reverse.

Good luck!