I used Andrea Fortuna's advice.
I used the terminal.
You need to be root.
# 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 upgrade # apt-get dist-upgrade
The whole procedure took me about 3 hours.
$ lsb_release -a No LSB modules are available. Distributor ID: Debian Description: Debian GNU/Linux 9.0 (stretch) Release: 9.0 Codename: stretch