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.
Mine: 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
Also Mine: (On Order) 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