BMW exhaust tips are usually snap riveted. The rivets need to be drilled out and removed then the exhaust tip can be pulled off. The muffler will need to be dropped or removed to get to all of the rivets. The replacement tips can be sourced from number of places, both OEM and aftermarket. The replacement tip is then secured with new snap rivets.
A muffler shop may be able to do this for you.
His: 2016 M4 Coupe | Sakhir Orange Metallic | Black Full Merino Leather | CF Trim | M DCT | Adaptive M Suspension | 19" Black Alloy Wheels | ZEC | ZLP | ZDB | MPE | ED 5/13/16
Hers: 2011 335is Couipe | Le Mans Blue Metallic | Black Dakota Leather | Dark Glacier Aluminum | 6MT | ED 5/12/11
Also His: 2006 Z4 Roadster (E85) | Interlagos Blue Metallic | Black Extended Nappa Leather | Carbon Leather Trim | Purchased 7/19/12