I'm not sure whether you mean "Default" filters that come with P6 or the "Global" filters available to all users. I'll respond to both.
You can't edit the "Default" filters that were supplied by the installation of P6. However you can copy the "Default" filters to your user section and see what is inside them. Then you can change them and save them as "Global" filters that are available to all users.
The "Global" filters are those that are available for all users. There is a Global privilege called "Add/Edit Global Activity and Assignment Layouts and Filters" that if checked in the Global Profile assigned to your users will allow you to maintain Global Filters.