Its simply a round rubber ring, I believe you can gently pull out the sensor (not by the cable) twist it gently. The rubber ring will come out and the new one POP in.
Item number three on this page
http://www.realoem.com/bmw/enUS/showparts?id=LM53-USA-06-2010-E89-BMW-Z4_30i&diagId=11_3737