I see most recent patch realeased MAGENTO 2.2.3, 2.1.12 and 2.0.18 SECURITY UPDATE
I want to download for my Magento version 2.1.9. How and where do I download the patch file?
You can download them from official Magento website:
https://magento.com/tech-resources/download
Some more information:
https://magento.com/security/patches/magento-223-2112-and-2018-security-update
If you have ssh access to your magento instalation, you can update magento using composer:
composer require magento/product-community-edition 2.2.3 --no-update
And then run:
composer update --optimize-autoloader
bin/magento setup:upgrade
bin/magento setup:di:compile
bin/magento setup:stati-content:deploy
Note: Make a backup before preceed. All comands must be executed on magento root folder.