Raspbian Buster is available and, if like me, you cannot currently afford to upgrade to a Raspberry Pi 4 and you don’t want to install Buster from fresh then it is time to do an in place operating system (OS) upgrade.
I am going to doing this in place upgrade on my Raspberry Pi 3, currently running Raspbian Stretch.
Note: Before doing an upgrade it is worth backing up any files/data from the Pi just in case the upgrade goes wrong.
There are two files that I am altering so that Raspbian will know about Buster. I am going to use Nano , if you have a preferred text editor (not a word processor) then feel free to use that.

The first file can be edited via:
sudo nano /etc/apt/sources.list
and the reference to Stretch on the uncommented line will need changing to Buster and then CTRL X, Y, Return to save and exit from the file. sources.list should end up looking like this:

The second file can be edited via:
sudo nano /etc/apt/sources.list.d/raspi.list
and like sources.list the reference to stretch on the uncommented line should be edited to read buster and then CTRL X, Y, Return to save and exit from the file. sources.list should end up looking like this:

With the sources.list locations updated enter:
sudo apt-get update
sudo apt-get -y dist-upgrade
Raspbian Stretch will now start the upgrade to Raspbian Buster. This upgrade process can take a bit of time (stick the kettle on and a make brew) and the install will ask some questions about configuration depending on what packages, applications or configuration is already in place in Stretch.
After the installation and it’s reboot the OS version can be checked using the line:
lsb_release -a

And if the upgrade has been successful then Codename: should read buster.
If an in place OS upgrade is not what you want, or if you are looking to start with a fresh install then check out https://www.raspberrypi.org/downloads/raspbian/ to download Buster.
Congrats on courage to upgrade, and success. Just out of curiosity, did you upgrade a “Real Stretch” or “Stretch upgraded from Jessie”?
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It was a Stretch image that I installed (fresh) back in March 2019 and has been used regularly since then.
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I’ve gone through the process of upgrading Stretch (and it was probably a Stretch upgraded from Jessie) to Buster, everything worked fine, but lsb_release is still showing Stretch, even on rebooting a couple of times.
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