ISO file readonly?

ISO 9660 is by design a read-only file system. This means that all the data has to be written in one go to the medium. Once written, there is no provision for altering the stored content. Therefore ISO 9660 is not suitable to be used on random-writable media, such as hard disks.

You need to copy whole directory tree to another directory, make your changes and then burn a new image.

If you happen to run an OS supporting it (Linux & BSDs), you might use Unionfs to somewhat mount an ISO in read-write mode. All writes will be actually be done on a read-write file system but depending on your needs, that might fit them.