Date: 2022feb3 Q. CentOS: Convert from Centos 8 to Centos Stream 9 A. With a browser visit http://mirror.stream.centos.org/9-stream/BaseOS/x86_64/os/Packages/ and download:

centos-gpg-keys-9.0-9.el9.noarch.rpm centos-stream-release-9.0-9.el9.noarch.rpm centos-stream-repos-9.0-9.el9.noarch.rpm

As root, remove the old packages

rpm -e --nodeps centos-linux-gpg-keys centos-release centos-repos

As root, install the packages you just downloaded:

rpm -Uvh centos*.rpm

Then do a big update:

dnf --allowerasing update

This worked for me but probably isn't the official way.