Thermostat downstairs in rental unit controls heat upstairs. Is there anyway to over ride so I have control upstairs without complete overhaul of unit? I am the owner with office up & rental down.

Just have a professional change the wiring and control to the upstairs. Or you can put a thermostat guard/lock assembly around the thermostat downstairs. That way the temp will be set and the thermostat locked so the downstairs folks have no access to the thermostat only you will have the key.