How do I disable Mid Stairs. Doesn't look to cool on a B737 with pax coming out of the engine.
I don't know what "B737" you are referring to. Assuming you mean the PMDG 737, I can assure you the configuration included with GSX doesn't use Mid Stairs on any variant.
So, clearly, if you see the middle stairs on a 737, it's means either you enabled them yourself (so just disable them by editing the relevant doors), or you must have downloaded a profile that clearly is wrong for a 737 so, just RESET the configuration, so GSX will use the one with supply it with. That is, assuming with "B737", you mean the PMDG, of course.