Yet another button mapping noob

Thanks, Seal.
I’ve sucessfully mapped the picade to match this diagram:

Here’s a concrete scenario of what I’d like.
I have Atari 2600 Pitfall.
Right now, the upper-yellow button (#1 - “B”) is the “jump” button.
Let’s say I want “jump”, for Pitfall only, to be the lower-yellow button (#4 - “A”).
How do I do that?

I’m thinking it’s in the runncommand menu that pops up right before the game starts, option #9 “Edit custom RetroArch config for this ROM”.

But what do I put in there, this?:

Finally, how do people map their hotkey to the Picade, when all the keys there are already spoken for?
There’s got to be a better way to quit a game that hitting a key on the keyboard + the start key on the Picade.

Thanks for all your help!