... But I think - someone correct me if I am wrong - when one "upgrades" from ANY earlier version, the only option through Apple is its latest OS (which, currently, is High Sierra).
The only way I know of to go back to an earlier version is if one has access to a copy of that previous version (e.g. El Capitan) which was made on a separate bootable drive prior to installing said version.
Wrong. Urquhart's advice and URLs worked perfectly for me — as I imagine they would for anyone.
