How to upgrade Debian v6(Squeeze) to v7(Wheezy) to v8(Jessie)

edit source.list
vim /etc/apt/sources.list

Debian for v6(Squeeze) to v7(Wheezy)
deb wheezy main
deb-src wheezy main

deb wheezy/updates main
deb-src wheezy/updates main

# wheezy-updates, previously known as 'volatile'
deb wheezy-updates main
deb-src wheezy-updates main

Debian for v7(Wheezy) to v8(Jessie)
deb jessie main
deb-src jessie main

deb jessie/updates main contrib
deb-src jessie/updates main contrib

deb jessie-updates main contrib
deb-src jessie-updates main contrib

upgrade 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


#check version after reboot
cat /etc/debian_release
lsb_release -a
uname -a

How to sucessfully upgrade Debian8 to Debian9 using VestaCP

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