There is no such thing like upgrade from CMS admin, admin is only for administration purpose. Kentico recommends to using upgrade tool read the documenation. You will have to do the upgrade for each major version, i.e. 9 ->10, 10->11, 11->12. At the end you can just go to client.kentico.com and upgrade your license from 9 to 12 (but that is just a button clicks).