Switch on the ignition but dont start the engine.
Hold down the trip computer reset button by the gauges. This will bring up a menu in the instrument cluster detailing each service.
Select the one you wish to reset by repeatedly pressing the trip reset button.
Once you have highlighted the brake interval, hold the trip reset button down. It will then ask if you want to reset, then press and release the trip reset button.
Turn the ignition off and back on. Check it has reset in the I-drive menu.
Failing this, or if the interval is too high, it will have to be reset using BMW test equipment.