A very simple writeup to update Debian 8 (Jessie) to Debian 9 (Stretch) is available here:
https://linuxconfig.org/how-to-upgrade-debian-8-jessie-to-debian-9-stretch or
https://www.rootusers.com/how-to-upgrade-debian-8-jessie-to-debian-9-stretch/

Cheers

For successful usage of zephir-lang on Debian, make sure to install the xml extension of PHP, as the compilation function within zephir uses some php functions which are only available if XML extension is installed.

Cheers