That looks to be the thermostat housing.
Shouldn't be too difficult depending on your skills. You will need the torx head tools to undo it.
Drain radiator then remove housing
You may as well replace the thermostat which is under the housing while you are doing it. When you buy the thermostat then you should get the correct seal or gasket for it and refit it all.
Get some more radiator inhibitor for refilling.
Once done and refilled you need to run the engine up to temp with the radiator cap off and your heater on hot to bleed any air from the system.