The under body cover is held on by a bunch of 8mm screws, about 22-24 IIRC. A couple of the screws are actually on the side of the wheel well housing and may not be readily apparent.
You'll have to put the car up on jacks (or on a lift if you're lucky!). Use a power screwdriver or a drill bit adapter in an electric drill to make it a lot easier to unscrew/rescrew them. You may be able to start at the front and remove enough screws to pull the cover down and hopefully the light cover will slide down the cover to the front.
His: 2016 M4 (F82) | Sakhir Orange Metallic | Black Full Merino Leather | CF Trim | M DCT | Adaptive M Suspension | 19" Black Alloy Wheels | ZEC | ZLP | ZDB | ED 5/13/16
Also His: 2006 Z4 Roadster (E85) | Interlagos Blue Metallic | Black Extended Nappa Leather | Carbon Leather Trim | Purchased 7/19/12
Hers: 2011 335is Coupe (E92) | Le Mans Blue Metallic | Black Dakota Leather | Dark Glacier Aluminum | 6MT | ED 5/12/11