Please post more details.
For updating in the version 18 you must use the gui or the command line.
On command line:
sudo -u www-data php /var/www/html/nextcloud/update/updater.phar
(change username and path to updater.phar)
Please post errors in gui and/or command line.
https://docs.nextcloud.com/server/18/admin_manual/maintenance/update.html