== == The easiest way is to syphon all the fuel from the tank using a old second hand fuel pump. You will need minimum, 1 old 12v fuel pump, get from a scrap yard etc.
15ft fuel hose that will fit both ends of the pump.
And some crocodile clips and wiring "at least the length of car" to power up the pump.
And some empty fuel drums or oil drums etc. This applies to virtually all cars, however some are easier than others to do. On the BMW lift out the rear seats you should see access to the fuel tank, depending on the BMW their may be one or two "if it's a saddleback tank", carefully take off the cover and lift out as best as possible the in tank pump.
Stick in your pipe and power up the pump drain it into the tanks you have, until you have reached the bottom of the tank, if you have a saddleback tank do the same on the other side. Replace all covers and seats etc, now put in about £10 worth of the RIGHT fuel. If the car has not been started or driven since the wrong fuel went in then it should start up and run ok. If the car has been run you will have to remove the fuel line from the engine "try and catch fuel in a container or add some of your drain pipe" to stop possible fuel ignition on hot engines, and crank over for 10/20 seconds until the wrong fuel is flushed out of the lines, reconnect up and start engine. GOOD LUCK.