Pretty simple really....radiator cap off, lower rad hose on water pump off, let it drain out, refill, job done.
If you want to do a proper flush, remove the reservoir and flush it, fill the system with fresh water, run the engine till up to temp and drain again. Refill with your prefered coolant ....job done.
I recommend you don't use the BMW coolant, use a sulphate/silicate free coolant....just my opinion. I personally believe it's the cause of some of the water pump seal failures.
Smithy.