About License under software manager
That is about the text of the license (GPL v3) itself, not the software. It means that while the FSF allows you to use the GPL license for your projects (which often involves copying the text of the license when you distribute the software), you cannot modify the text of the license itself. The license itself is owned by the FSF and they do not give anyone the right to modify it. You are free to modify the software, of course, as long as you obey the GPL when redistributing your changes.