Previously it was a three step process to upgrade from CentOS Linux 8
(CL8) to CentOS Stream 8 (CS8). We have reduced it to a two step
process by adding the centos-stream-repos package to the Extras repo
in CL8. You can now upgrade from CL8 to CS8 by running just these two
commands.

[root@centos ~]# dnf swap centos-linux-repos centos-stream-repos
[root@centos ~]# dnf distro-sync

The instructions have been updated on the main website and Carl George has recorded a short screencast to demonstrate the process.

source
centos 8