easy upgrade to debian 9

I used Andrea Fortuna's advice.

I used the terminal.
You need to be root.

system upgrade

# apt-get update
# apt-get upgrade
# apt-get dist-upgrade

backup of current sources.list

cp /etc/apt/sources.list /etc/apt/sources.list_backup

 package repository update

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

package index update

apt-get update 

upgrade process

# apt-get upgrade
# apt-get dist-upgrade

reboot

reboot 

The whole procedure took me about 3 hours.

result:

$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 9.0 (stretch)
Release: 9.0
Codename: stretch