[CentOS] Upgrade from Centos 7 to Centos 8

Wed Apr 8 15:50:44 UTC 2020
Stefano Simonucci <stefanosimonucci.sim at alice.it>

I don't understand how I can upgrade my CentOS 7.

cat /etc/redhat-release
CentOS Linux release 7.7.1908 (Core)

I followed the directions found on the web (by installing dnf and so 
on), but when I tried to install centos-release I get

dnf upgrade -y 
http://mirror.centos.org/centos/8/BaseOS/x86_64/os/Packages/centos-release-8.1-1.1911.0.9.el8.x86_64.rpm
Last metadata expiration check: 0:19:05 ago on Wed 08 Apr 2020 04:50:14 
PM CEST.
centos-release-8.1-1.1911.0.9.el8.x86_64.rpm 184 kB/s |  21 kB     00:00
Dependencies resolved.

  Problem: cannot install the best update candidate for package 
centos-release-7-7.1908.0.el7.centos.x86_64
   - nothing provides centos-gpg-keys = 8.1-1.1911.0.9.el8 needed by 
centos-release-8.1-1.1911.0.9.el8.x86_64
   - nothing provides centos-repos = 8.1-1.1911.0.9.el8 needed by 
centos-release-8.1-1.1911.0.9.el8.x86_64
=================================================================================================================================
  Package                         Arch 
Version                             Repository Size
=================================================================================================================================
Skipping packages with broken dependencies:
  centos-release                  x86_64 
8.1-1.1911.0.9.el8                  @commandline 21 k

Transaction Summary
=================================================================================================================================
Skip  1 Package

Nothing to do.
Complete!

Does anyone know how to upgrade?

Thanks

Stefano