The radiator and the engine block can be drained very easily.
Remove the plastic piece that covers the bottom of the engine.
Loosen the blue plastic screw that allows the radiator to drain into a 4 quart pot (the screw is at the bottom of the radiator and on the driver's side).
Slide under the engine on the passenger side, look up at the 2nd cylinder from the front, just in front of the catalytic converter, and you will see a bolt (maybe 12 or 13 mm) that you will loosen. Radiator fluid will pour all over so get out the way quickly. Hardly any will go in the pot but have a pot handy anyway.
Replace the block bolt, use a fresh 14mmx18mm aluminum washer, and gently tighten the blue plastic screw.
Add 3 quarts of BMW antifreeze.
Add 3+ quarts of DISTILLED water. Use a large funnel. This is slow. Start the engine and turn the heater switch on to full-hot when you are about beginning to add the 2nd quart of water.
Open the bleeder screw if there is one. Burp the air out by lightly squeezing the top radiator hose. You can feel the air bubbles as little jolts when they burp through the hose.
When the level of the fluid is right, and you don't feel or see air bubbles, you are done. TIghten the bleeder screw gently. It is flimsy.
All this takes about 45 minutes. The capacity of the cooling system is about 8.5 quarts, so obviously some of the old fluid remains in the system. That is ok, but if you want to flush it all out you have to add a cycle or 2 of adding only DISTILLED water before you get to the antifreeze stage.
I like to add RedLine Water Wetter too.