assume your talking about a cosworth? - i had the exact same problem as you on the weekend; started the engine and the temp gauge was in the red

the temp sensor is at the fron of the head on exhasut side with a single wire connected to it.

take single wire off, you'll need 1/2 inch spanner to undo the sensor, refit a new one and job done, you will loose a little water so have the new one in your hand ready!

refill header tank if need be

PS the sensor is white from memory

reconnect wire....job done