51 21... Removing and installing left or right lock barrel of front door

Necessary preliminary tasks: 

• Remove CARRIER FOR OUTER DOOR HANDLE 

Release screws (1).

Remove gearing cover (2).

Fig 1: Identifying Gearing Cover And Screws

Courtesy of BMW OF NORTH AMERICA, INC.

Press down latch mechanism (1) slightly.

Turn lock cylinder (2) in direction of latch mechanism (1) and remove.

Fig 2: Turning Lock Cylinder

Courtesy of BMW OF NORTH AMERICA, INC.