In some cases, we have installed a stable version of Debian or other similar distribution and we want to upgrade it without having to reinstall it, for compatibility or lab scenario reasons.
The only thing we need to do is to change ‘source.list’ file, replacing ‘stable’ to ‘testing’:
su - nano /etc/apt/sources.list deb http://deb.debian.org/debian testing main contrib non-free deb-src http://deb.debian.org/debian testing main contrib non-free
Later, we can update the repositories, upgrading to testing branch and remove any superseded packages:
apt-get update && apt dist-upgrade -y && apt autoremove -y