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:


wget -P /etc/apt/trusted.gpg.d https://ftp-master.debian.org/keys/archive-key-11-security.asc


wget -P /etc/apt/trusted.gpg.d https://ftp-master.debian.org/keys/archive-key-11.asc

How To Upgrade to Debian 11 Bullseye

Edit your /etc/apt/sources.list like this:

deb http://deb.debian.org/debian bullseye main
deb-src http://deb.debian.org/debian bullseye main

deb http://deb.debian.org/debian-security/ bullseye-security main
deb-src http://deb.debian.org/debian-security/ bullseye-security main

deb http://deb.debian.org/debian bullseye-updates main
deb-src http://deb.debian.org/debian bullseye-updates main

2.) Update to Debian

apt update

apt dist-upgrade

3.) Reboot

After this reboot into the upgraded OS and new kernel that came with it.

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