How do I back up my data?

There no back up in nextcloud setting. I don’t know how to back up my data. Can’t even find. My nextcloud account is not even administration.

Because of “clouddamo” in your subject i think you use Cloudamo at your hoster. If you use a Managed Nextcloud you can pay money and the Nextcloud hoster makes the backups for you. Some Nextcloud hoster include backup in there package.

Normally for Nextcloud backup you need the backup of the server data and server configuration, the user configuration and the database which contains e.g. contacts and shares.

I think the easiest way to have your own backup (data) is to use e.g. the Windows Nextcloud client. Keep in mind that this is a synchronization. Errors will be synchronized, too. Therefore, you can regularly backup the contents of your Nextcloud client e.g. to an usb device or second place.