The thermostat is in the thermostat housing, where three big rubber coolant hoses come together next to the radiator towards the driver's side of the front of the engine block. It is held together by a few bolts (10mm maybe?). Drain some coolant out , open the housing, change the thermostat, then refill and bleed the system. Should not take that long, just make sure enough coolant is drained otherwise there will be a very big mess when you open the housing.
If your engine is not reaching operating temp, there is no other possible culprit besides the thermostat.