Step 1.) Upgrade to Debian 11 first
The process to go to Debian 12 is not as smooth as 11, when trying to upgrade from Debian 10. In fact, it doesn't work directly, so you'll first need to follow this guide to update to Debian 11, reboot and come back here if successful.
Step 2.) Update sources.list
Update your /etc/apt/sources.list like this:
deb http://........
This likely works for even older versions but I have only tested on 8,9,10 (11,12). It's quite impressive at how easy it is to upgrade from a very old version to the new version. I would say that Debian version upgrades are some of the quickest and smoothest of any distro.
1.) Backup your /etc/apt/sources.list
cp /etc/apt/sources.list ~
#Get your keys first, you need the latest keys for Debian 11 or it won't work:........
MySQL on Debian versions is configured differently than the native local MySQL plugin so you will be disappointed when your password on the mysql client fails by default.
Here is how you reset the MySQL root password the proper and "working way"
#first we gracefully stop mysql
sudo systemctl stop mysql;
#then we forcefully kill any mysqld process just in case
sudo killall -9 mysqld mysqld_safe;........