apart from any particular circumstances you haven't mentioned, there's generally no need to update the cpan tool/script outside of it getting updated together with updating perl, whenever you do that. so what you're probably talking about is updating the list of sites that cpan tool uses to search/download modules. there's generally no need for that either, unless your cpan tool is not working when you try to search/download perl modules. you can update the list manually when you enter cpan tool from command line without specifying parameters, and follow the inbuilt help for this. Or are you running your own little CPAN mirror that you want to update?
please be more specific.