How do you change the thermostat on a 89 Chevy pickup 1500?

Should be located in a 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

Clean both surfaces

Install new thermostat and gasket

Re-fill coolant slowly to prevent an airlock

Start engine and look for leaks

Run engine to normal operating temperature with heater on

Re-check coolant level