Took me a while to find out how, too. Finally looked in the manual. I kept looking for a release button on the headrests themselves to do a manual adjustment. I should have known it would be electrical/electronic.
2005 545i, Jet Black, Black Leather, Cold Weather pkg, Logic7, 6 disk changer, Sport Package, Automatic, moonroof, electric rear sun shade, dark wood trim, PDC, Xenon Adaptive lights, Nav, Sat radio, 245/40 ZR18 - 275/35 ZR18 tires, Progman 30.02