autoexec.bas files

Maybe borh

LOAD "AUTOEXEC.BAS"
to load the code.
Then:
LIST
to list the source.
Or add line numbers to show just a range of it:
LIST 10-150

To edit a line, just list it, and make the changes you want, and hit ENTER key on it to commit the changes to memory.
To insert a line, type a new line by starting it with the number that's in between the lines you want to insert it at, followed by a space and the code.

Once you are happy with all your changes, you can save it to disk:
SAVE "AUTOEXEC.BAS"

Plenty of books on The Internet Archive if you want to learn more about MSX BASIC: https://archive.org/search?query=subject%3A%22MSX+BASIC%22