There are a couple ways to alter the .cfg file:
Until you are comfortable altering the .cfg, I'd advise always making a backup of the original before modifying an copy...I rename the original file aircraft_orig.cfg and keep it in the same folder as aircraft.cfg.
However, hiding original values behind double slash marks will preserve original values in the aircraft.cfg copy that you are currently using/modifying...for reference, also you can save changes and notations that way for reference.
Change weight method 1. "Remove seats and misc weight by reducing the aircraft's empty_w