To Change Mirror Manually You have to change your mirror address in /etc/apt/sources.list

After that run the sudo apt update command

Bardia Moshiri Mirror in Iran

For Debian 12

deb http://mirror.bardia.tech/debian bookworm main non-free contrib non-free-firmware

deb http://mirror.bardia.tech/debian bookworm-updates main non-free contrib non-free-firmware

deb http://mirror.bardia.tech/debian-security bookworm-security main contrib non-free non-free-firmware

For Debian 11

deb http://mirror.bardia.tech/debian bullseye main non-free contrib

deb http://mirror.bardia.tech/debian bullseye-updates main non-free contrib

deb http://mirror.bardia.tech/debian-security bullseye-security main contrib non-free

For Debian 10

deb http://mirror.bardia.tech/debian buster main non-free contrib

deb http://mirror.bardia.tech/debian buster-updates main non-free contrib

deb http://mirror.bardia.tech/debian-security buster-security main contrib non-free non-free contrib

For Debian 9

deb http://mirror.bardia.tech/debian stretch main

deb http://mirror.bardia.tech/debian stretch-updates main

deb http://mirror.bardia.tech/debian-security stretch-security main contrib non-free non-free contrib