The airbag light normally comes on if you disconnect the airbag or the seat electrical connectors and then turn the ignition on. Perhaps you disconnected a side airbag connector?

Anyway, you can use Carsoft or any of the other BMW diagnostic tools (INPA, GT1, DIS, PA Soft) cheaply available via eBay to turn it off (and do lots more diagnostics besides). Otherwise there are cheap tools also available which can reset the airbag light.