The sensor needs to be removed when the Nico app notifies you that the sensor has expired and needs to be replaced.
To remove it, simply pull up the edge of the adhesive that keeps your sensor attached to your skin. Make sure to firmly grip it. Slowly peel away from your skin in one motion.
After removing the sensor, make sure to clean and disinfect the area, and let it air-dry completely, before applying a new sensor.