Upgrade Server 9 to Server 12

is it possible to upgrade from nextcloud server 9 to server 12?
Can this be done through the Update App?
Or where can I find a procedure for this?

Have a look at https://docs.nextcloud.com/server/12/admin_manual/maintenance/upgrade.html and the manuals for previous versions.

I would do a manual upgrade from 9->10, not sure if the updater app recognize the version correctly. After NC 10 it should be ok with the updater app. The updater app should then also show which version is going to be installed (should be 10->11->12).