Unscrew the bolt in the block around the 4th cylinder. Not very comfortable to crawl, but it is possible. (key on 17-19). Also, Unscrew the plug on the radiator (large screwdriver) if not Unscrew, then remove the bottom outlet of the radiator. If the valve of the stove mechanical - open (turn the knob until it stops in the red zone). If the electric - nothing else to do. Assemble everything in reverse order.
To expedite the process - from the thermostat to remove the top pipe of the radiator (optional).
Loosen the clamp holding the bottom outlet of the radiator to the thermostat housing. Pour the liquid into the radiator until will flow under the bottom of the pipe and tighten the clamp of the lower pipe and put top. Pour liquid into the tank up to the level.
If the screw on the thermostat manages to Unscrew - you can purge any residual air. Start the engine, a few minutes drive, shut down, add more coolant. The next day - again check the level and top up as needed.
BMW m20 motor was used on cars:
BMW 3 - E21, E30 - 320, 323, 325
BMW 5 - E12, E28 - 520, E34 - 520, 525