Howto Upgrade Debian 4 Etch to Debian 5.0 Lenny
HowTo Upgrade Debian 5.0 (Lenny) To Debian 6.0 (Squeeze)
HowTo Upgrade Debian 6.0 Squeeze to 7.0 Wheezy
HowTo upgrade to Debian 8 Jessie from Debian 7 Wheezy
The command you need to know:
upgrade is system level and the dist-upgrade is the entire level.
# aptitude upgrade
# aptitude install
# aptitude dist-upgrade
# apt-get update
# apt-get install apt dpkg aptitude
# apt-get dist-upgrade
#############Debian 4 Etch
deb http://archive.debian.org/debian-archive/debian/ etch main non-free contrib
deb-src http://archive.debian.org/debian-archive/debian/ etch main non-free contrib
#############Debian 5.0 Lenny
deb http://archive.debian.org/debian-archive/debian/ lenny main non-free contrib
deb-src http://archive.debian.org/debian-archive/debian/ lenny main non-free contrib
########### Debian 6.0 (Squeeze)
deb http://mirrors.163.com/debian/ squeeze main non-free contrib
deb http://mirrors.163.com/debian/ squeeze-proposed-updates main non-free contrib
deb-src http://mirrors.163.com/debian/ squeeze main non-free contrib
deb-src http://mirrors.163.com/debian/ squeeze-proposed-updates main non-free contrib
########### Debian 7.0 Wheezy
deb http://mirrors.163.com/debian/ wheezy main non-free contrib
deb http://mirrors.163.com/debian/ wheezy-proposed-updates main non-free contrib
deb-src http://mirrors.163.com/debian/ wheezy main non-free contrib
deb-src http://mirrors.163.com/debian/ wheezy-proposed-updates main non-free contrib
############Debian 8 Jessie
deb http://mirrors.163.com/debian/ jessiemain non-free contrib
deb http://mirrors.163.com/debian/ jessie-proposed-updates main non-free contrib
deb-src http://mirrors.163.com/debian/ jessiemain non-free contrib
deb-src http://mirrors.163.com/debian/ jessie-proposed-updates main non-free contrib
when upgrade from the old one to the new one, step by step to change the source.list and run upgrade/dist-upgrade command,
the smart system will upgrade it step by step as well. please find the right source and it will run quit well and fast for you.
good luck.