You can calibrate the temperature based on a reliable reference such as for instance an analogic thermometer. You need to put your reference and the Netatmo modules close to each other for an adequate amount of time to allow the measurements to stabilize.
You can add an offset (+/- 5°C) to the temperature:
- Go to Settings > Manage my Home
- Select the room where the module is located
- Click on the module
- In Configurations select Temperature
- Enter the value in the corresponding field