please fill out https://retropie.org.uk/forum/topic/3/read-this-first when seeking support.
you're on the right track - you need to edit the file. this is the way to change the hotkey combos.
I have tried the file manager in retropie but when I put in sudo nano /opt/retropie/configs/all/retroarch-joypads/Microsoft X-box 360 pad.cfg
the file comes up blank.
wrong path. it will be within /opt/retropie/configs/all/retroarch/autoconfig
what is returned if you input the following into the command line?
ls -l /opt/retropie/configs/all/retroarch/autoconfig
i am guessing your path is
/opt/retropie/configs/all/retroarch/autoconfig/Microsoft\ X-box\ 360\ pad.cfg
note you need slashes for files with spaces in them, or you can surround the whole path with quotes)
also, you shouldn't use for sudo for this kind of thing. no need.