Surprised no one chimed in yet. Pull it up until it reaches its maximum point. Then at the base of the headrest that's attached to the seat, you'll see a sort of notched plastic piece that you push in while removing the headrest.
Similar to the piece to the bottom right seen here:
2011 e92 JzB 6MT
Akrapovic Evolution, ESS Tune, UUC SSK w/ DSSR, Stoptech SS brake lines, Eibach springs, OEM gloss black kidney grills, keyhole delete, LUX H8 3.0, full front clear bra, 20% tint, Macht Schenll Stud Conversion, Apex ARC-8 18x9.5/10 w/ 265/35 & 285/30 AD08's