It's very easy. Select the plant where you want to put the sensor and click on the sensor icon. Choose "associate with sensor" and then click the button on the Wi-Fi Plant Sensor to identify the device. It might take a few seconds until the change applies.
We advise to wait two water cycles before changing the sensor from one plant to the next (Koubachi has to tell you two times to water your plant). In this way the models are surely well adapted to your plant's location.