Debian 10 was just released and upgrading from Debian 9 isn’t as complicated as you might think.

Before you start: make sure you have backups and that any packages/software you use also has support for Debian 10 (otherwise it might be wise to wait).

First, create a backup copy of the /etc/apt/sources.list file. 

sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak

Then open the file for editing (replace nano with your editor of choice). 

sudo nano /etc/apt/sources.list

Change all references from stretch to buster (example below). 

deb http://ftp.se.debian.org/debian debian buster main

Make sure you also update any other apt sources found in /etc/apt/sources.list.d directory (if your listed repositories has buster supported).

Update the local package list.

sudo apt update

Then update all packages.

sudo apt upgrade -y

Finally, you can run the distribution upgrade.

sudo apt dist-upgrade

Once finished, reboot your system. Running lsb_release -a (or hostnamectl) after reboot should display buster instead of stretch if performed correctly.

Congratulations! You now have a working installation of Debian 10.