Where is the thermostat located and how do you replace it on a Renault megane?

Its located in the thermostat housing. Follow the top hose from your radiator to it and you should find it. There will be 3 or 4 torx screws holding the housing in place remove them, you will lose water. Pull the housing aprt and you will see the thermostat, just pull it out and replace with new one don't forget the rubber gasket if it has one, then put back together and top up with water make sure to get the air out.