Different BMW models require a slightly different process to remove the navigation computer however all are generally the same. The following photos show removing the navigation computer in a BMW E38 7 Series

In the back of the car locate the navigation computer drive and insert 4 pieces of a cutup coat hanger or a "Radio Removal Tool" into the holes of the drive

Once the tools are in the holes you can grasp the navigation drive faceplate and slide it out

Photo shows closeup of where the tools release the locking mechanism