Actually this is more simple than you might think.
First you checkout your current version. Then download and un-tar the new version and simply run the following command on your shell/terminal:
rsync -av --exclude '.svn' --exclude '._*' my/new/cakephp-cakephp-efb6e08/ my-old-cakephp/
This will exclude your Subversion .svn directories and it will also exclude the META-data files created by Mac OS X.
Good luck with upgrading! 🙂