The Release Notes for Katello 3.9 point to upgrade instructions which are broken.
If you drop down for EPEL 7 (Centos etc) you get this:
yum update -y https://yum.theforeman.org/client//el7/x86_64/foreman-client-release.rpm
which errors with Cannot open: https://yum.theforeman.org/client/el7/x86_64/foreman-client-release.rpm. Skipping.
Looks like there’s a missing version number in that URI.
yum update -y https://yum.theforeman.org/client/1.20/el7/x86_64/foreman-client-release.rpm
Also, now that I’ve run that and found it doesn’t work because I’m not upgrading the foreman client - I need to install it apparently, that could be more clearly explained on the original release notes - it looks like the problem also exists on the installation notes page.