You need to factory reset the thermostat and set it up as yours.
First check which model of thermostat you have and then factory reset it:
How to tell which Nest thermostat you have - Google Nest Help
Restart or reset a Nest thermostat - Google Nest Help
Nest Thermostat
For the Nest Thermostat model, use the Google Home app. The QR code and setup key are physically printed on the back of the thermostat. Follow these steps:
Get started with your Nest Thermostat - Google Nest Help
Nest Thermostat E and Nest Learning Thermostat
For Nest Thermostat E and Nest Learning Thermostat models, use the Nest app. The QR code and setup key are in the settings on the thermostat itself and are not printed on the device. Follow these steps:
Add your Nest thermostat to the Nest app - Google Nest Help
