The only shell codes I know abut are Shell Scripts. Scripts are programs which run in interpreters. Shell scripts are the Unix and GNU/Linux equivalent of Windows bat files -- which means they are text files and of course editable.

Do you mean a "shell script" which runs in a shell? If so yes, these are usually just text files which you can edit with a text editor.

Or

Do you mean the shell executable (.e.g. /bin/bash) ?

If you want to modify the executable, perhaps to change the way the shell behaves, it depends on which shell. If it is an Open Source shell (such as bash), download the source, modify it, build you own version, and replace the standard executable with your executable.

If you are using some proprietary shell (like MS DOS), then you can't modify it because you won't have access to the source code.

But you can still edit shell script files that run under that shell.