Press ESC with the C64 window focused to stop the game

After that you can type LIST to see the code, and edit it, then RUN to restart

I have all 4 volumes of Input.

It's too bad Commodore 64 BASIC is horrible, and you basically have to learn all about ROM routines and machine language to do anything useful.

Yes and no. Yes, it stunk for BASIC programming, but it also meant that by necessity you made the jump to driving the hardware bare metal much quicker, which is the best way to get the most out of it.