If I'm not mistaken the licensing messages live outside the MediaWiki: namespace so can only be changed by a developer. When you're ready to do so, please file a request on bugzilla.wikimedia.org to change the licensing info.
I believe everything is in the MediaWiki namespace these days.
The notice on edit pages is MediaWiki:Copyrightwarning and MediaWiki:Copyrightwarning2. The notice at the bottom of each page is MediaWiki:Copyright.