You have three options :
1) Take it to a dealer or specialist and get stung.
2) Find someone with the right diagnostics kit who'll do it for you for beer money.
3) Find out if there's a combination of button presses on the dashboard that will reset it for you, often something like - "Hold trip meter button, turn ignition on, rub head whilst patting belly, left leg in, right leg out, turn ignition off."
Try searching for a BMW owners forum, someone is bound to have found out.
My VW actually has the procedure listed in the owners manual.