If during the update process, you encountered the below problem,
The following extra files have been found:
.cache
Update failed. To resume or retry just execute the updater again.
I found the solution 
do the step-by-step
1- login to the server
2- try to find .cache folder (ls -ahl)
3- sudo rm -rf .cache
4- sudo chown www-data:www-data /var/www/nextcloud/ -R
5- sudo -u www-data php --define apc.enable_cli=1 /var/www/nextcloud/updater/updater.phar
The update process is starting … and show you the next step