You can change a car thermostat if you follow the vehicle manual and instructions carefully. It’s also a good opportunity to replace the coolant and flush the cooling system to protect the new thermostat from contamination. However, a faulty thermostat is not the only cause of abnormal engine temperatures. There could be a problem elsewhere in the cooling system. Consider a low coolant level, leaks, a faulty water pump, worn water pump belt, faulty radiator fan, collapsed or leaking hose, or a clogged radiator.
To prevent potential expensive damage, it pays to have a full cooling system check done by a qualified technician when you observe unusual engine temperatures.