This software is copyright and you are explicitly granted a license which gives you, the "user" of the software, legal permission to copy, distribute and/or modify the software, so long as if you modify the software then it carry a different name from the original software.
All the same, please check the specific details of the software's license before making modifications.
Further information about this type of license is not currently available but general information is available about other TeX Catalogue Licenses.