How to upgrade Magento 2 to a newer version and install latest security patches

Note: Make sure to backup your store and dB before performing a Magento upgrade. 

Note 2: Backup your .htaccess file,as it is overwritten when you upgrade. 

1. Via SSH, issue the commands below in the root of your Magento installation. 

php bin/magento deploy:mode:set developer

composer require magento/product-community-edition 2.2.5 --no-update

composer update

rm -Rf var/cache/ var/generation/ var/page_cache/ var/view_preprocessed/ var/di;

php bin/magento setup:upgrade

2. Restore your .htaccess file from your backup

For more details about the upgrade process please check the Official Magento 2 Documentation