The #50 lens is held in its 90 degree positions by a dent in the plastic. If rotated in the proper direction it takes a substantial amount of pressure to release the lens from the dent. To rotate, hold the body of the sensor with both hands and use your thumbs to push up/down the opposing corners of the lens housing.
If the flat of the lens is towards the programming button on the sensor, then rotate the lens clockwise.
If the curved portion of the lens housing is towards the programming button on the sensor, then rotate the lens counter clockwise.
The above is true for both the RM(R) and CM(R)B sensor styles.