Are you upgrading Foreman or Foreman with Katello?
If Foreman without Katello, here is the official guide: https://theforeman.org/manuals/2.4/index.html#3.6Upgrade
We are building out docs.theforeman.org from a base of documentation that was once Red Hat Satellite and orcharhino by atix.de documentation. While the doc set is robust and has a lot of useful information, the primary scenarios covered have Katello included by default, as is the case for the enterprise solutions the docs came from. The Foreman without Katello installation and upgrade docs are not yet ready at that address, but work is ongoing.
Sorry for any confusion.