First, you need to get the appropriate elbow. The elbow from the M-Roadster works in an OBD-II engine, but it's missing a fitting for an OBD-I engine. If you do a search on here or Bimmerforums, you can probably find the correct part numbers.
After that, it's just a matter of loosening the clamps at each end of the ASC throttlebody, disconnecting any hoses, unhooking any cables, and taking the unit out. Then you install the appropriate elbow, reconnecting any hoses, and tucking the cables out of the way somewhere (I have them zip-tied to some other wires nearby).
If the ASC light bothers you, you can pull the gauge cluster and remove the bulb, or if you wait long enough, the bulb will eventually burn itself out
