Just upgraded successfully to Magento 2.2.6 through composer:

    composer require magento/product-community-edition 2.2.6 --no-update

Change the version accordingly e.g. 2.2.6

    composer update
    rm -rf var/di var/generation/* var/view_preprocessed/* var/cache/* var/page_cache

*// The issue following commands one by one

    chown -R user:group .
    chmod u+x bin/magento
    php bin/magento cache:flush
    php bin/magento setup:upgrade
    ph bin/magento setup:di:compile
    php bin/magento setup:static-content:deploy
    php bin/magento indexer:reindex

If getting permissions error, issue:

    chown -R user:group .
command again where user and group correspond to your server user and user group.