Parktronic sensor removal

[acme8850]

Hi,
Had a Star diagnostic done by my local independent.
Came up with a couple of faulty sensors that have been causing the Parktronic fault, amongst other things.
So now I know what is wrong the question is how do I get them out of the bumper to replace them? 1 front & 1 back.
The car is a 2000 E320CDI.

Any advice greatfully recieved.

Regards
Vern