• Put Nextcloud into Maintenance mode by adding this line in config.php which can be found in your nextcloud web directory.

<?php
 
 "maintenance" => true,

• Backup folder using rsync

sudo rsync -Aavx /where/your/web/data/is/nextcloud-data /media/whereyourbackupdriveis/nextcloud-dirbkp_`date +"%Y%m%d"`/

• Backup your dataase by using PHPmyadmin how to import/export can be found here