Hi Tato
If you are on 5.6.3.5 already your only upgrade option is to 5.6.4, there is no 'upgrade' mechanism to a higher version than 5.6.4.
To move to 5.7 or version 8. you must either migrate your content or rebuild your site from scratch on the new platform.
If you are upgrading to 5.6.4 so you can switch to PHP7, you will have to ensure your theme and addons are also PHP7 compliant.
I can help you with the above, just contact me via Private Message to establish communications between us.
Regards
Weyboat

Many thanks for your message Weyboat.

It looks like it's getting kind of complicated with c5. I m considering to shift to WP. I might need some help though, keep in touch and fantastic spring time!