vim /etc/apt/sources.listDebian for v6(Squeeze) to v7(Wheezy)
deb http://ftp.debian.org/debian/ wheezy main
deb-src http://ftp.debian.org/debian/ wheezy main
deb http://security.debian.org/ wheezy/updates main
deb-src http://security.debian.org/ wheezy/updates main
# wheezy-updates, previously known as 'volatile'
deb http://ftp.debian.org/debian/ wheezy-updates main
deb-src http://ftp.debian.org/debian/ wheezy-updates mainDebian for v7(Wheezy) to v8(Jessie)
deb http://ftp.debian.org/debian/ jessie main
deb-src http://ftp.debian.org/debian/ jessie main
deb http://security.debian.org/ jessie/updates main contrib
deb-src http://security.debian.org/ jessie/updates main contrib
deb http://ftp.debian.org/debian/ jessie-updates main contrib
deb-src http://ftp.debian.org/debian/ jessie-updates main contribupgrade Debian
apt-get update
apt-get upgrade -y
apt-get dist-upgrade -y
#if error “There is no public key available for the following key IDs”
#fix by http://taxze.com/?p=11
reboot#check version after reboot
cat /etc/debian_release
lsb_release -a
uname -aHow to sucessfully upgrade Debian8 to Debian9 using VestaCP
https://forum.vestacp.com/viewtopic.php?f=41&t=15452&p=63601
