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The thermostat is located on the engine block were the lower radiator hose conects to the block. You can drain the coolant out and remove the hose from the lower housing. You can then remove the housing by removing the two bolts that hold the housing to the block. You can use a long 5mm ball allen socket or a 10mm short socket with a swivel on it. Either will work. Once you replace the thermostat, and refill the coolant make sure that you have coolant flow on the small hose that goes to the coolant bottle, if not then you will need to clear out what ever is blocking the flow, failure to have flow here will cause overheating as the engine can not purge the air out of it.
Here is a picture of the system, see number 9 for the bolt and 10 for the housing.
Number 6 is were the lower radiator hose is attached to the block