1991 Chevy Corsica thermostat

With the engine cold, take the radiator cap off, then follow the coolant hose on the top of the radiator all the way until it meets the engine, where it meets is called the coolant inlet housing, that is where your thermostat is, take those two bolts off, you'll have a little coolant rushing out, dont worry, pop the old thermostat out and put the new one in, but before you put the new one in make sure any crud is removed from the housing surface. Then after you put the new one in replace the housing and bolt it up nice and tight and your good to go