Is there a fee or license required to modify the software?

There is no fee to modify the software.  Any modifications to to the software must be made under the terms of the Apache Version 2.0 license. Any modifications or redistributions of the software must provide proper attribution. Apache Version 2.0 license is a permissive free software license meaning you are not required to share back your modifications with the community or release them under the Apache License. However, we encourage all contributions to be shared back with the community so that other can benefit your innovation and help to collectively maintain it.  Please see the Apache License FAQs for more on on the license.