BIAB 2016 Mac, unable to find a way to make a melody change in middle (bridge) of song. don't see mention of melody in Bar Settings ??

Your safest bet is to set up the midi file in a dedicated midi editing software (often referred to as DAW) and import the completed file back into BBox.

OK Thanks, to confirm then, it can not be done in BIAB !

If it can, then I in my many years of using the program on my Mac, like I presume you are using, have not come across the way.
So no, I don't think you can.