Hi Juergen,
remove the clipped big outer chrome ring from the headlamp assembly, starting at the lowest point.
If someone replaced the headlamp bowl/complete unit by a screwed version instead if the clipped one , remove the possible single screw at the lowest point of the big outer chrome ring
Remove the 3 screws keeping the small inner chrom ring in place.
Remove the small chrome ring together with the reflector/headlamp insert.
Take care, after you remove the last of the 3 screws keeping the small chrome ring, both parts (ring and glas) can fall down......
Insert a continental glas unit, attach the small chrome ring with the 3 screws.
Adjust the headlights or let a workshop adjust them using the two adjustment screws.
Assemble the big outer chrome ring after adjustmeht is done
Uwe