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 https://mirrors.tuna.tsinghua.edu.cn/debian buster main
deb https://mirrors.tuna.tsinghua.edu.cn/debian buster-updates main
deb https://mirrors.tuna.tsinghua.edu.cn 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