I'm using Ckan 2.2.1 . I need a responsive Ckan theme so after reading the change log http://docs.ckan.org/en/latest/changelog.html, it's possible on version 2.3 . Can i upgrade my actual instance or do i have to install a new Ckan instance?
Yes. You can upgrade actual instance. Check Upgrading CKAN article. You will need to restart solr too, because CKAN 2.3 uses new schema.xml version.