HOW DO CHANGE THE THERMOSTAT?

Well you could have a blown head gasket or faulty bottom hose or radiator cap, according to your unusual description. Simply follow the bottom radiator hose, unbolt the the 10mm bolts and remove the thermostat housing. Replace seal, refit everything, fill with 2 bottles of BMW coolant and water, bleed the system by cracking the bleed screw on the thermostat housing until all the air has escaped the system and check for leaks after a hose down and drive.