Upgrade it

sed -i 's/stretch/buster/g' /etc/apt/sources.list
sed -i 's/buster/bullseye/g' /etc/apt/sources.list

Debian 11

deb http://deb.debian.org/debian/ bullseye main contrib non-free 
deb http://deb.debian.org/debian/ bullseye-updates main contrib non-free
deb http://deb.debian.org/debian bullseye-proposed-updates main contrib non-free
deb http://deb.debian.org/debian-security/ bullseye-security main contrib non-free
deb http://deb.debian.org/debian/ bullseye-backports main contrib non-free

Debian 10

deb http://deb.debian.org/debian buster main
deb-src http://deb.debian.org/debian buster main
deb http://deb.debian.org/debian-security/ buster/updates main
deb-src http://deb.debian.org/debian-security/ buster/updates main
deb http://deb.debian.org/debian buster-updates main
deb-src http://deb.debian.org/debian buster-updates main

Debian 9

deb http://deb.debian.org/debian stretch main
deb-src http://deb.debian.org/debian stretch main
deb http://deb.debian.org/debian-security/ stretch/updates main
deb-src http://deb.debian.org/debian-security/ stretch/updates main
deb http://deb.debian.org/debian stretch-updates main
deb-src http://deb.debian.org/debian stretch-updates main