It's not a matter or being root, isofilesystems are read-only and you will likely get an error to that effect if you try to save the file. I think there may be an option for rw when you mount but I've not tried that. I usually just copy the mounted directory to my /home/user directory, make the changes and use mkisofs to create a new iso.
Or simply unmounting it would be sufficient.
I believe you would need to generate a new iso file.