This article explains a system upgrade procedure from Debian 9 Stretch to Debian 10 Buster.

Currently, Debian Buster is in a state of testing. The stable Debian 10 Buster release date is to be expected in a second or third quarter of 2019.

Buster Full Upgrade

Let's first fully upgrade our current Debian Stretch system:
# apt-get update
# apt-get upgrade
# apt-get dist-upgrade

Update Package Repository to Debian Buster

# sed -i 's/stretch/buster/g' /etc/apt/sources.list
Your /etc/apt/sources.list should look similar to the one below:
deb http://httpredir.debian.org/debian buster main
deb http://httpredir.debian.org/debian buster-updates main
deb http://security.debian.org buster/updates main
Once the above /etc/apt/sources.list file edit is completed, use apt-get command to update packages index:
# apt-get update

Upgrade to Debian Buster

The following below commands will upgrade your Debian 9 Stretch to Debian 10 Buster release.
# apt-get upgrade
# apt-get dist-upgrade
Once ready check your Debian version.