Upgrade Mint Linux from 19 to 20:

sudo apt update
sudo apt upgrade
sudo shutdown -r now
sudo apt install mintupgrade
sudo mintupgrade check
sudo mintupgrade upgrade
sudo reboot

Upgrade Debian from 10 (buster) to 11 (bullseye):

sudo apt update
sudo apt upgrade
sudo apt full-upgrade
sudo apt autoremove
sudo shutdown -r now
sudo cat /etc/os-release

Replace buster with bullseye and make a change to the security section.
Where the security section had “buster-updates”, it should be changed to “bullseye-security”.
Note: You may be using a different mirror.

sudo vi /etc/apt/sources.list

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

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