You can’t edit the en-us files as there is no files for it.
However the strings in the PHP files (en-us) will work as IDs for other languages. So if you need to edit the en-us string you need to edit the PHP file that contain that string.
But there is a better workaround and will save your work with any future updates.
Simply add a new language like en-uk and then make it as default language for your system and disable the en-us language from admin panel your users will not know it’s en-us or en-uk they will simply see the English title for your language.
Check this to learn how to add new language: