Replacing a thermostat is a relatively easy task. If your temperature gauge is running way down on the low side and your heater isn't putting out much heat, it's time to replace the thermostat. Here is a description of how to change a thermostat. The exact location of your thermostat may be slightly different but most of them are near the top radiator hose in the top of the engine.
The thermostat is normally in a housing which is at the engine end of the top radiator hose. Drain the coolant into a drain pan - toss it down the john - it's the correct way to dispose of it. Take a screwdriver and remove the hose clamp from the top hose where it meets the top of the engine. Get a flashlight and see if you can peek down the hole and see the thermostat. If it is faulty it will be open.
Not all thermostat housings are identical but typically it will be held on with two bolts. Remove them. Tap the thermostat housing with a wooden block or a soft faced hammer. It should pop off easily. You can then see the thermostat. Lift it out. Don't sweat the gasket - you are going to replace it anyway. Use a new gasket. Scrape EVERYTHING off both mating surfaces. Use a single edged razor blade. Apply some blue rubber sealant to both sides of the gasket. Then grab it between your fingers and "squeegee" any excess off. You don't need a lot of sealant.
Make sure you put the gasket on the right side of the thermostat. The stat will normally fit into a recess in the engine, the gasket goes on top and the housing on top of that. Wipe a bit of blue rubber sealant inside the hose, put it back onto the housing nipple and clamp it back in place.
You should probably drain all of the coolant by draining the block but if you can't find the small plug it the side of the engine don't sweat it. You should mix antifreeze and water in a ratio of 50-50 and replace what you drained out with the fresh mix. If you can't get all the replacement mix in while it's cold just warm it up a bit and watch the level drop as the thermostat opens up. Then add mix until it is totally filled. Replace the radiator cap and put a quart or so in the overflow bottle (which you should have drained previously).
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