I believe you are looking for the /media/realroot. There you can find all the contents stored in the SD card. In your computer, when you put in the SD card, a folder automatically appears with the contents of the SD card. In this folder you can view and edit the contents stored in the card.
You have several choices I assume from your question you are using Windows.
You can use SSH to connect and edit the files - I like to use Nano which is in AlexT's repository opkg update, opkg install nano, nano filename
Another great tool is filezilla, FileZilla - The free FTP solution that lets you copy/edit files easily
And my students who use Windows like using notepad ++ (Notepad++ Home) which allows you to edit files on a remote system using the familiar Notepad interface.