You need to know which module has the issue a code reader will display which is at fault, there are three, main module in centre console behind handbrake and 2 Satellite modules behind the side trim panels at the side of the seats SBSL and SBSR modules. You also need to know the part numbers so you can source a compatable working used replacement. Best do a parts search on Realoem parts site on your vehicles Vin Number then view the list of modules that are exchangable with your part number. Any replacement will require coding to your car with your Vin number otherwise the airbag light will remain lit up.
https://www.realoem.com/bmw/index.do
Put last seven letters and digits of your Vin in and then go to the section that lists the modules and part numbers.