How many Debian versions exists?

There are three different Debian versions namely stable, testing, and unstable.

• Debian stable is actual state of the distribution.
• Debian testing is like the stable version but has packages that came from the unstable in wich there were no bugs found in the last 10 days.
• Debian unstable contains not fully tested packages.

Debian testing will be the next stable version.

More info about DebianTesting can be found in the Debian Wiki.

Upgrade to Testing

cd /etc/apt/
mv sources.list sources.list.orig

echo "deb http://deb.debian.org/debian/ testing main contrib non-free" > sources.list

sudo apt update
sudo apt upgrade
sudo apt dist-upgrade

or you can do this (change buster with your actual version)

cd /etc/apt/
cp sources.list sources.list.orig

sed -i 's/buster/testing/g' /etc/apt/sources.list

sudo apt update
sudo apt upgrade
sudo apt dist-upgrade

finally

reboot