PHP settings are set using the nginx configuration file, here: /etc/nginx/sites-available/default.
Simply change the line containing upload_max_filesize=50M and post_max_size=50M to any suitable same value, then save and restart the nginx and php7.0-fpm service, like that:
sudo systemctl restart nginx php7.0-fpm
or reboot the MoodleBox.
Caution: don't mess anything in this file. You're on your own ;-)