Finish.

Check Version.

update the package repository, enter the following command

To upgrade to version 9 of Debian, need to edit sources.list file

You should see many "jessie" in list, and please replace the content following

deb http://httpredir.debian.org/debian stretch main contrib non-free
deb http://httpredir.debian.org/debian stretch-updates main contrib non-free
deb http://security.debian.org stretch/updates main contrib non-free

And do it again.

apt-get update

apt-get upgrade

apt-get dist-upgrade ---> Please be careful. Yes or No.

apt-get autoremove

Done.