location of the engine sensor to change it

I have had my thermostat changed, the housing, motor mounts, spark plugs, timing belt, and water pump changed. I am getting an error code p0138. It is not overheating. It has plenty of coolant in it so I think the sensor needs to be changed since it started coming on after I got everything replaced except the spark plugs. Don't have money to pay anyone to change it and don't know where it's located to do it myself...HELP