How?

I'm not sure about the 9960, but for most models OFA does not document the method to clear to factory defaults. They document a clear method that only clears some of the settings.

On typical models the factory default level clear is done by a long press of the programming (Set or Magic or whatever) key followed by keying in 9 8 1.

I'm sure details of clearing the 9960 are described in some thread in this forum.