How do you replace the thermostat on a car?

All depends on the make and model of the car... But, most vehicles have the thermostat located where the top radiator hose is connected to the engine. There will a fitting bolted to the engine, and the top radiator hose connects to the top of it. The theromostat in inside of it... Since you didn't specify, this is for most American vehicles - the foreign cars and some of the new domestic cars can be different...

Once you find the thermostat housing, usually held on by two bolts, it is a simple matter of draining some about a 1/2 gallon of coolant out, removing the thermostat housing, removing the thermostat and old gasket, cleaning the gasket surfaces, installing the new thermostat and gasket, and bolting the housing back on, and refilling the radiator with fresh coolant...

Of course, before you start doing any work on the engine, you need to let it cool - near boiling coolant is bad news! Have the parts bought prior to starting to work on the car, so the motor/coolant stays cool. Also, it is easier to clean the gasket surfaces if you remove the top radiator hose from the thermostat housing, and replace when the thermostat installion is complete...

The thermostat housing will have a diaghram of which way the thermostat needs to face, and where the gasket goes - BUT pay attention to how it is installed when taking it apart - much easier than trying to figure out which way it goes later...

Good luck...