First, while engine is COLD, and while gauge is PEGGED, unplug the sensor and make sure the gauge drops to Zero. This will ensure it is indeed the sensor, and not grounded wiring somewhere. If it is the sensor:
-Buy a new Coolant temp sensor (less than 50 bucks if I recall)
-buy/borrow a torque wrench and a 1" deep socket w/extensions- if you dont have them already
-Make sure engine is ICE cold
-pull hoses and wiring out of the way on the driver side front of enging head, directly under the throttle body on the Driver side
-pop off electrical connector
-unscrew sensor
-reinstall new sensor, and Torque to (? Will check when I get home)
-plug in wiring, and check to make sure you have no leaks after starting the engine and letting it come to temp. check function of gauge, you should be good to go. Top off coolant if necessary after the engine cools back down.