You will either have to go to a BMW dealer or a specialist or mechanic who has a device that can reset this for you. But first, the reason as to why this light has come on will have to be diagnosed.
Normally with BMW's, its the harness under the passenger seat for the passenger occupancy sensor or seat belt pre-tensioner has become disconnected. Very common problem on BMW's.
If this has happened, I would advise to have the connector removed and the wires soldered together before resetting the light. Most mechanics are familiar with this as like I said, its common to BMW's
Either that you could buy yourself one of these
[broken link removed]
where are you based? I can recommend a mechanic in Sallins that fixed mine a year or so ago