I too felt like I was missing something. I had to look at a video on edmonds 3 times before I saw it and then my fumbling had purpose and presto - that was easy. I have a 2000 focus, driver side (left) head lamp lost the low beam.
1. Unscrew the cap to expose the problem area.
2. There is the "

" spring which wraps around the unit. On the upper left (about 10 o'clock facing the front) where you can't see it (perhaps you can with a mirror), the spring ends in a round loop (say 3/8 of an inch). Use the unhook move on the round loop end. That is a push down and up (towards 12 o'clock) move to unhook it. Then it will "swing out of the way".
Very simple. Impossible to see. You should be able to feel the loop end. I felt like Mr. STUPID, so thought I would come back and spread the joy.
BTW, my US Focus battery does not get in the way of my left side lamp cover.