The steps to remove the dash to get to the HVAC box are in the BMW TIS. I could try to write that up, but the TIS explains it fairly well.

What I could not find in TIS was what to do once you get down to the big black pastic box enclosing the evaporator. I suspect BMW buys it as a unit from a supplier.

I can try to write up some steps if you want to try the repair yourself.