Not a common question since the light isn't enabled on US vehicles. This link shows how to reset the other service interval lights; I would scroll through the menu options to see if you have an option for the brake fluid.

How to reset the Service Lights on a BMW X5 - Xoutpost.com

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue