There appears to be a couple of missing links in the 12.0 installation guide. The text at https://www.freebsd.org/releases/12.0R/installation.html says: The procedure for doing a source code based update is described in and .
A commit references this bug: Author: gjb Date: Fri Feb 22 23:08:57 UTC 2019 New revision: 52828 URL: https://svnweb.freebsd.org/changeset/doc/52828 Log: Fix broken link to "Updating FreeBSD from Source" chapter. PR: 235947 Submitted by: bcran Sponsored by: The FreeBSD Foundation Changes: head/en_US.ISO8859-1/htdocs/releases/12.0R/installation/installation.xml
Fixed in r52828. Thank you for the report.
this earlier report can be closed then https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=213523
(In reply to Vitalio from comment #3) wrong link above. Sorry. The right one is https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=224952
*** Bug 224952 has been marked as a duplicate of this bug. ***