In order to upgrade your version of wordpress via linux command line perform the following steps.
curl -O https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar
mv wp-cli.phar wp-cli
chmod u+x wp-cli
/root/wp-cli --allow-root core update --path=/path-to-wordpress-install
This will perform an update as a root user. Typically you may not want to use a root user so you can run as any user desired. You can also add this to crontab to happen on a regular basis