if you can access your server have you tried commandline update?
i trust this way than through the web updating, command is sudo -u www-data php /var/www/nextcloud/updater/updater.phar
good luck
I also had this problem. Probably your download got stuck. What helped for me was deleting/moving a hidden file .../nextcloud/data/updater-ocxxxxx/.step
Then you can start upgrade process again.
I had to do this several times. Maybe there is an overload on the download servers…