Replacing 12 V sockets:

Open cover (1) of socket housing.

Turn socket insert (2) approx. 30° to right.

Turn socket insert (2) under tension back into initial position and pull out of socket housing.

Fig 1: Opening Cover Of Socket Housing

Courtesy of BMW OF NORTH AMERICA, INC.

Installation: 

Align socket insert (1) with retaining lugs (2) to groove (3) of socket housing and slide in until it is felt snapping into place.

Fig 2: Aligning Socket Insert With Retaining Lugs To Groove Of Socket Housing

Courtesy of BMW OF NORTH AMERICA, INC.

Pull socket housing (1) approx. 5 mm out of wheel arch trim (2) and tilt out to right at bottom.

Installation: 

Insert socket housing with plastic lugs (3) into wheel arch trim and lock with retaining lugs (4).

Fig 3: Pulling Socket Housing

Courtesy of BMW OF NORTH AMERICA, INC.