How do you replace the thermostat in a 1994 Pontiac Firebird?

Should be located in the removable housing at the engine end of the upper radiator hose Drain about 1 gallon of coolant from the radiator

Remove this housing

Note how thermostat is installed in housing

Clean both surfaces

Replace thermostat and gasket

Re-fill coolant slowly to prevent an air lock

Start engine and look for leaks

Run engine to normal operating temperature with heater on

Re-check coolant level