Removing the sensor is like removing a band-aid. It might sting a bit, but you shouldn't experience any real pain.
To make it easier to remove, try doing so after taking a warm shower as this will soften your skin.
Remove the bandage that covers your sensor.
Pull up the edge of the adhesive that attaches the sensor to your skin.
Slowly peel away the sensor in one motion.
Dry the surface of the sensor if it is wet with a clean, dry tissue.
Place it inside the return packaging.
You can remove any remaining sticky residue on your skin using warm, soapy water.
You may find that you have small pin pricks of blood on your arm where the sensor was stuck on. This is completely normal as the filament goes right underneath your skin. If this happens, use an alcohol wipe to clean the area and add a normal band aid to keep it covered.