There are 2 types of xenon headlights both slightly different fitting.
- Remove the rubber cap.
(old inside the headlight)
- Disconnect the green cable from the side of the pack.
- Unscrew the pack
- Unscrew the outer plastic housing ring
- Remove the bulb
(new inside the headlight)
- Disconnect the green cable from the side of the pack.
- Unscrew the pack
- Remove the bulb
Im not sure when during the 172 production they decided to switch to the new units and remove the need for the outer plastic housing but its no issue if you have it or dont really.