If the thermostat iis stuck on open, do I need to replace the whole thermostat?

Firstly let’s prove the thermostat is indeed “stuck open” - symptoms = temperature gauge always indicating cold engine - heater doesn’t put out warm air. I see you added “Not overheating but air conditioning not cold” .. That’s not an indicator the thermostat is stuck open - that is more likely an a/c problem. Lastly if you’ll Google thermostat (for your vehicle) you will see what the replacement looks like on the resulting page. Good luck!