You can end a sensor early by stopping the sensor in Settings > CGM. Once you have stopped the sensor in app, remove the transmitter from the sensor cradle and let the transmitter rest for 15+ minutes before placing a new one.
For more information about how to replace your sensor, check out How do I replace my sensor?
CAUTION: Once stopped, the same sensor cannot be restarted or reused. Once stopped, you should remove the sensor from the transmitter and then throw the sensor away. Do not throw away your transmitter, as it can be reused.