Yes, I had to flip the belt over on the driver side of my 560SL, no idea how it happened, but somehow the belt's male buckle was the wrong way round and it was really annoying to have a twist in the part coming out of the shoulder-height socket....
Anyway, take off the paneling and expose the retractor at the floor. Pull the belt all the way out until you can see the slot where it is attached to the cylinder. As I recall, I rigged up vice grip pliers (and a screwdriver wedge, I think) to hold the spring loaded cylinder stable, with the slot facing up so it was easy to work on.
You need a thin screwdriver to work the metal plate out of the slot, which is what retains the belt on the cylinder. Once this is out of the slot, you can flip the belt over, or put in a different belt with the same size plate.
Sorry I can't remember the details of how I clamped the cylinder while working on it, but you'll figure it out I'm sure.

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1986 560SL
2002 Toyota Camry
1993 Lexus