Hi
I still have a version 5 instance running and wanted to upgrade, but where have the related documents been moved to?
I did this operation on another instance during last summer, but the URL is no longer valid 
https://doc.owncloud.org/server/6.0/admin_manual/maintenance/update.html#upgrade
Any change to keep the content of my actual database (mysql) within the new version?
Hi,
the upgrading documentation is available here:
https://doc.owncloud.org/server/latest/admin_manual/maintenance/manual_upgrade.html
Depending on the amount of your users and files i wouldn't risk to upgrade such an outdated version. Mostly its easier, safer and faster to backup your data on client side, install a fresh oC 9.1.3 instance and re-import your data from client side (after you have recreated your user).
The main issue is that you would need to do a upgrade over 7 major releases step by step:
latest 5.0.x -> 6.0.x -> 7.0.x -> 8.0.x -> 8.1.x -> 8.2.x -> 9.0.x -> 9.1.x
As you might guess this is a lot of effort and no one can guarantee that this is working at all.