Upgrade from 15 to 18

The update process is described here:

https://docs.nextcloud.com/server/18/admin_manual/maintenance/upgrade.html?highlight=upgrade

or here

IMPORTANT: Please have an eye on the supported PHP versions of the major releases. Very often users try to use v7.2/.3/.4 with older Nextcloud versions which hadn’t supported it at the original release date :wink:

https://docs.nextcloud.com/server/18/admin_manual/installation/system_requirements.html