You can reset the sensor by pushing the button for at least 10 seconds. The LED will turn red for a short time to indicate that the configuration has been reset. After this, your credentials on the sensor will be deleted. To reuse it later on you will have to reconfigure Wi-Fi Plant Sensor.
Please note that this process is usually not necessary and will not help in case of problems. It is only useful if you want to give your plant sensor away and you want to wipe your Wi-Fi credentials before doing so.